Image forming apparatus

ABSTRACT

An image forming apparatus includes a laser beam scanning unit adapted for scanning laser beams on a surface of a charged image bearing member ( 5 ). The laser beam scanning unit ( 53 ) comprises a semiconductor laser array ( 21 ) having a plurality of light emitting portions ( 21   a ) and a rotating polygon mirror adapted for deflecting laser beams emitted from the light emitting portions to the image bearing member ( 5 ). The light emitting portions ( 21   a ) are disposed two-dimensionally on a surface of the semiconductor laser array. The lighting and the amount of light of the light emitting portions ( 21   a ) are discretely controlled by a control unit. The scanning unit further includes a collimator lens having a focal length fc, where a distance between a most spaced two of the plurality of light emitting portions is .delta. max, and fc/.delta. max is 25 or more.

This application is a divisional of application Ser. No. 08/878,900filed on Jun. 19, 1997 now U.S. Pat. No. 5,870,132, which is acontinuation of Ser. No. 08/466, 602 filed on Jun. 6, 1995 nowabandoned, which is a divisional of Ser. No. 07,971,908 filed on Dec.18, 1992 now 5,610,647 which is the national stage of InternationalApplication PCT/JP92/00620 filed on May 14, 1992 and which designatedthe U.S., claims the benefit thereof and incorporates the same byreference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus for scanninglaser beams and thereby forming a latent image on a image bearingmember.

RELATED ART

Heretofore, a large number of image forming apparatuses for forming astatic latent image on a image bearing member with a laser beam and forprinting the image on a paper at a high speed by a electrophotographicprocess have been used as output units of computers, facsimile machines,multi-functional copy machines, and so forth. In recent years, there hasbeen an urgent need for improving the output speeds of theseapparatuses. Accordingly, the apparatuses have been actively improved.

For example, an image forming apparatus using a rotating polygon mirrortype deflecting unit deflects one laser beam with each facet and drawsone scanning line. Thus, to increase the number of scanning lines in aparticular time period, provided that the number of small mirrorsurfaces of the rotating polygon mirror is constant, the number ofrotations should be increased. On the other hand, provided that thenumber of rotations is constant, the number of mirror surfaces of therotating polygon mirror should be increased. To increase the number ofrotations of the rotating polygon mirror, a dynamic or static bearingusing pneumatic or hydraulic power is required. These bearings areexpensive and difficult to handle. Thus, they are difficult to use inconventional laser printers. In contrast, provided that the number ofmirror surfaces of the polygon mirror is increased, since the deflectionangle becomes small, the length of the optical path following thedeflecting unit becomes large. In addition, the diameter of collimatedlaser beams entering into an image forming optical system becomes largein proportion to the length of the optical path. Thus, the sizes of thelens and the rotating polygon mirror become large. In particular, when ahigh resolution is required, since the number of scanning lines areincreased, the number of rotations of the polygon mirror and the lengthof optical path should be further increased. This situation also appliesto the case where the deflecting unit is not a rotating polygon mirror.In this case, the scanning frequency and the length of the optical pathfollowing the deflecting unit increase. To solve these problems, anexposing technique for writing a plurality of scanning lines with aplurality of laser beams in one scanning sequence has been developed.This technique is referred to as a multi-beam exposing technique.

To obtain a plurality of laser beams, a plurality of gas laser (forexample, He—Ne) oscillators are used as a light source. In addition, atechnique wherein a laser beam generated by one oscillator istime-divided into a plurality of portions by an acousto-opticalmodulator (AOM) or the like has been developed. As a technique forsimplifying the construction of the unit and decreasing the sizethereof, for example, as disclosed in Japanese Patent Laid-OpenPublication Serial No. SHO 54-7328, a semiconductor laser array where aplurality of light emitting portions for radiating laser beams areintegrally disposed on one device is used as a light source.

Next, an image forming apparatus using a semiconductor laser array willbe described. An image forming apparatus uses a laser array integrallydisposed on one substrate as a light source. A beam radiating point ofeach light emitting portion is disposed at an edge of a semiconductordevice substrate. A plurality of laser beams are collimated by a commoncollimator lens so that they have a particular diameter. Thereafter, thecollimated laser beams are introduced into one facet of the rotatingpolygon mirror (deflecting unit). As the facet rotates, the laser beamsare deflected. Next, the laser beams are spotted as an image through animage forming lens. Then, the image spot exposes the image bearingmember and thereby a static latent image is formed. Next, in accordancewith the electrophotographic process, the latent image is developed,transferred onto a paper, and then fixed. In addition, as disclosed inJapanese Patent Laid-Open Publication Serial No. SHO 54-158251, todecrease the distance of each adjacent scanning line scanned on theimage bearing member at a time, the light emitting portions of the laserarray are disposed at particular angles to the scanning plane.

On the other hand, there is still another need of an image formingapparatus using such a semiconductor laser array which can scan laserbeams at a high speed and with a high resolution. However, theconventional image forming apparatus has not satisfactorily accomplisheda high-speed and high-resolution laser beam scanning technique.

DISCLOSURE OF THE INVENTION

The present invention has been made with consideration of the abovedescribed points. An object of the present invention is to provide acompact image forming apparatus for scanning laser beams at a high speedand with a high resolution.

A first feature of the present invention is an image forming apparatus,comprising a image bearing member for forming a static latent imagethereon, a charging unit for charging the surface of the image bearingmember, a laser beam scanning unit for scanning a plurality of laserbeams on the surface of the image bearing member which is charged, and adeveloping unit for causing a developing agent to adhere on the surfaceof the image bearing member scanned with the laser beams, wherein thelaser beam scanning unit comprises a semiconductor laser array having aplurality of light emitting portions for emitting laser beams, the lightemitting portions being formed on a device substrate, and a deflectingunit for deflecting laser beams emitted from the light emitting portionsto the surface of the image bearing member, wherein the light emittingportions are disposed two-dimensionally on a surface of thesemiconductor laser array, and wherein the lighting and the amount oflight of each of the light emitting portions are discretely controlled.

A second feature of the present invention is a laser beam scanning unit,comprising a semiconductor laser array having a plurality of lightemitting portions for emitting laser beams, the light emitting portionsbeing formed on a device substrate, and a deflecting unit for deflectingthe laser beams emitted from the light emitting portions, wherein thelight emitting portions are disposed two-dimensionally on a surface ofthe semiconductor laser array, and wherein the lighting and the amountof light of each of the light emitting portions are discretelycontrolled.

A third feature of the present invention is an image forming apparatus,comprising a image bearing member for forming a static latent imagethereon, a charging unit for charging the surface of the image bearingmember, a laser beam scanning unit for scanning a laser beam on thesurface of the image bearing member which is charged, and a developingunit for causing a developing agent to adhere on the surface of theimage bearing member scanned with the laser beam, wherein the laser beamscanning unit comprises a semiconductor laser having a light emittingportion for emitting a laser beam, the light emitting portion beingformed on a device substrate, and a deflecting unit for deflecting alaser beam emitted from the light emitting portion to the surface of theimage bearing member, and wherein the light emitting portion has anoptical axis substantially perpendicular to the surface of the devicesubstrate.

A fourth feature of the present invention is a laser beam scanning unitcomprising a semiconductor laser having a light emitting portion foremitting a laser beam, the light emitting portion being formed on adevice substrate, and a deflecting unit for deflecting the laser beamemitted from the light emitting portion, wherein the light emittingportion has an optical axis substantially perpendicular to a surface ofthe device substrate.

A fifth feature of the present invention is an image forming apparatuscomprising a image bearing member for forming a static latent imagethereon, a charging unit for charging the surface of the image bearingmember, a laser beam scanning unit for scanning a plurality of laserbeams on the surface of the image bearing member which is charged, and adeveloping unit for causing a developing agent to adhere on the surfaceof the image bearing member scanned with the laser beams, wherein thelaser beam scanning unit, comprising a semiconductor laser array foremitting a plurality of laser beams, a collimator lens for collimatingeach of the laser beams, a deflecting unit for periodically deflectingthe direction of each of the laser beams collimated by the collimatorlens, and a scanning lens for imaging each of the laser beams deflectedby the deflecting unit on the image bearing member, and wherein thedeflecting unit is a rotating mirror with one reflecting surface.

A sixth feature of the present invention is a laser beam scanning unit,comprising a semiconductor laser array for emitting a plurality of laserbeams, a collimator lens for collimating each of the laser beams, adeflecting unit for periodically deflecting the direction of each of thelaser beams collimated by the collimator lens, and a scanning lens forforming the laser beam deflected by the deflecting unit on a imagebearing member, wherein the deflecting unit is a rotating mirror havingone reflecting surface.

A seventh feature of the present invention is an image formingapparatus, comprising a image bearing member for forming a static latentimage thereon, a charging unit for charging the surface of the imagebearing member, a laser beam scanning unit for scanning a plurality oflaser beams on the surface of the image bearing member which is charged,and a developing unit for causing a developing agent to adhere on thesurface of the image bearing member scanned with the laser beams,wherein the laser beam scanning unit, comprising a semiconductor laserarray having a plurality of light emitting portions for emitting aplurality of laser beams, a collimator lens for collimating each of thelaser beams, a deflecting unit for periodically deflecting the directionof each of the laser beams collimated by the collimator lens, and ascanning lens for imaging each of the laser beams deflected by thedeflecting unit on the image bearing member, and wherein the followingrelation is satisfied

fc/δ max>25

where fc is the focal length of the collimator lens and δ max is thedistance between the mutually most spaced-apart two of the lightemitting portions of the semiconductor laser array.

An eighth feature of the present invention is a laser beam formingapparatus, comprising a semiconductor laser array having a plurality oflight emitting portions for emitting laser beams, a collimator lens forcollimating each of a plurality of laser beams, a deflecting unit forperiodically deflecting the direction of each of the plurality of laserbeams collimated by the collimator lens, and a scanning lens for imagingthe laser beams deflected by the deflecting unit on the image bearingmember, wherein the following relation is satisfied

fc/δ max>50

where fc is the focal length of the collimator lens and δ max is thedistance between the mutually most spaced-apart two of the lightemitting portions of the semiconductor laser array.

A ninth feature of the present invention is an image forming apparatus,comprising a image bearing member for forming a static latent image on asurface thereof, a charging unit for charging the surface of the imagebearing member, a laser beam scanning unit for scanning a plurality oflaser beams to the charged surface of the image bearing member, and adeveloping unit for causing a developing agent to adhere on the surfaceof the image bearing member, wherein the laser beam scanning unitcomprises a plurality of light emitting portions adapted for emittinglaser beams and disposed on a device substrate, and a deflecting unitfor deflecting the laser beams emitted from the light emitting portions,wherein the center axis of each laser beam emitted from thesemiconductor laser array is substantially perpendicular to a surface ofthe device substrate, wherein an aperture stop is disposed at a positionwhere cross sections of the laser beams are matched at least partially,the position being on an optical path between the semiconductor laserarray and the deflecting unit, and wherein, provided that the strongestpower of laser beams which has passed through the aperture stop is 1,the powers of the remaining laser beams are 0.9 or higher.

A tenth feature of the present invention is a laser beam formingapparatus, comprising a semiconductor laser array having a plurality oflight emitting portions for emitting laser beams, the light emittingportions being disposed on a device substrate, and a deflecting unit fordeflecting laser beams emitted from the light emitting portions, whereinan aperture stop is disposed at a position where cross sections of thelaser beams are matched at least partially, the position being on anoptical path between the semiconductor laser array and the deflectingunit, and wherein, provided that the strongest power of laser beamswhich has passed through the aperture stop is 1, the powers of theremaining laser beams are 0.9 or higher.

An eleventh feature of the present invention is an image formingapparatus, comprising a image bearing member for forming a static latentimage on a surface thereof, a charting unit for charging the surface ofthe image bearing member, a laser beam scanning unit for scanning aplurality of laser beams on the charged surface of the image bearingmember, and a developing unit for causing a developing agent to adhereon the scanned surface of the image bearing member, wherein the laserbeam scanning unit comprises a semiconductor laser array having aplurality of light emitting portions for emitting laser beams, acollimator lens for collimating laser beams emitted from the lightemitting portions, and a deflecting unit for deflecting the laser beams,wherein an aperture stop is disposed on an optical path midway betweenthe semiconductor laser array and the deflecting unit, and wherein thefollowing relations are satisfied$\frac{st}{f} \leq {{0.12\quad ( \frac{D}{d} )^{2.3}} + 0.17}$$\frac{D}{d} \leq 2$

where f is the focal length of the collimator lens, s is the distancebetween a deflecting unit side focal point of the collimator lens andthe aperture stop, t is the distance between a light emitting portionspaced farthest from an optical axis of the collimator lens and theoptical axis, D is the diameter of the aperture stop, and d is thediameter of each of the collimated beam.

A twelfth feature of the present invention is an image formingapparatus, comprising a image bearing member for forming a static latentimage on a surface thereof, a charging unit for charging the surface ofthe image bearing member, a laser beam scanning unit for scanning aplurality of laser beams on the charged surface of the image bearingmember, and a developing unit for causing a developing agent to adhereon the scanned surface of the image bearing member, wherein the laserbeam scanning unit comprises a semiconductor laser array having aplurality of light emitting portions for emitting laser beams, acollimator lens for collimating laser beams emitted from the lightemitting portions, and a deflecting unit for deflecting the laser beams,wherein an aperture stop is disposed on an optical path midway betweenthe semiconductor laser array and the deflecting unit, and wherein thefollowing relations are satisfied$\frac{st}{f} \leq {{0.06\quad ( \frac{D}{d} )^{2.9}} + 0.08}$$\frac{D}{d} \leq 2$

where f is the focal length of the collimator lens, s is the distancebetween a deflecting unit side focal point of the collimator lens andthe aperture stop, t is the distance between a light emitting portionspaced farthest from an optical axis of the collimator lens and theoptical axis, D is the diameter of the aperture stop, and d is thediameter of each of the collimated beam.

A thirteenth feature of the present invention is a laser beam scanningapparatus, comprising a semiconductor laser array having a plurality oflight emitting portions for emitting laser beams, a collimator lens forcollimating the laser beams emitted from the light emitting portions,and a deflecting unit for deflecting the laser beams, wherein anaperture stop is disposed on an optical path midway between thesemiconductor laser array and the deflecting unit, and wherein thefollowing relations are satisfied$\frac{st}{f} \leq {{0.12\quad ( \frac{D}{d} )^{2.3}} + 0.17}$$\frac{D}{d} \leq 2$

where f is the focal length of the collimator lens, s is the distancebetween a deflecting unit side focal point of the collimator lens andthe aperture stop, t is the distance between a light emitting portionspaced farthest from an optical axis of the collimator lens and theoptical axis, D is the diameter of the aperture stop, and d is thediameter of each of the collimated beam.

A fourteenth feature of the present invention is a laser beam scanningapparatus, comprising a semiconductor laser array having a plurality oflight emitting portions for emitting laser beams, a collimator lens forcollimating the laser beams emitted from the light emitting portions,and a deflecting unit for deflecting the laser beams, wherein anaperture stop is disposed on an optical path midway between thesemiconductor laser array and the deflecting unit, and wherein thefollowing relations are satisfied$\frac{st}{f} \leq {{0.06\quad ( \frac{D}{d} )^{2.9}} + 0.08}$$\frac{D}{d} \leq 2$

where f is the focal length of the collimator lens, s is the distancebetween a deflecting unit side focal point of the collimator lens andthe aperture stop, t is the distance between a light emitting portionspaced farthest from an optical axis of the collimator lens and theoptical axis, D is the diameter of the aperture stop, and d is thediameter of each of the collimated beam.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a laser scanning optical system ofa first embodiment of an image forming apparatus in accordance with thepresent invention;

FIG. 2 is a side view showing the image forming apparatus;

FIG. 3 is a sectional view showing an optical resonator of a surfacelight emitting type semiconductor laser array;

FIG. 4 is a perspective view showing a light emitting portion of aphase-lock surface-light-emitting type semiconductor laser array;

FIG. 5 comprises schematic diagrams showing the relation of scanninglines and spot positions;

FIG. 6 comprises schematic diagrams showing an optical resonator of alight emitting portion of the phase-lock surface-light-emitting typesemiconductor laser array;

FIG. 7 is a schematic diagram showing the conception of a conventionaledge emitting type semiconductor laser array;

FIG. 8 is an optical side view showing an optical path of a conventionallaser scanning optical system;

FIG. 9 is a schematic diagram showing the relation of scanning lines andspot positions;

FIG. 10 is a graph indicating reflectances of P and S polarization of aconventional metallic mirror;

FIG. 11 is a schematic diagram showing a method of adjusting thecollimate diameter of a laser beam;

FIG. 12 is a schematic diagram showing a laser scanning optical systemof a second embodiment of the image forming apparatus in accordance withthe present invention;

FIG. 13 is a side view showing the image forming apparatus;

FIG. 14 is a sectional view showing an optical resonator of a surfacelight emitting type semiconductor laser;

FIG. 15 is a perspective view showing a light emitting portion of aphase-lock surface-light-emitting type semicondutor laser array;

FIG. 16 comprises schematic diagrams showing an optical resonator of thelight emitting portion of the phase-lock surface-light-emitting typesemiconductor laser array;

FIG. 17 is an optical side view showing a scanning line of aconventional laser scanning optical system and an optical axisperpendicular to the scanning line;

FIG. 18 is a schematic diagram showing a conventional semiconductorlaser;

FIG. 19 is a graph indicating reflectances of P and S polarization of aconventional metallic mirror;

FIG. 20 is a schematic diagram showing a laser scanning optical systemof a third embodiment of the image forming apparatus in accordance withthe present invention;

FIG. 21 is a side view showing the image forming apparatus;

FIG. 22 is an optical side view showing an optical path on a scanningplane;

FIG. 23 is a sectional view showing an optical resonator of a surfacelight emitting type semiconductor laser;

FIG. 24 comprises schematic diagrams showing the relation of scanninglines and spot positions;

FIG. 25 is a plan view showing a beam deflecting unit;

FIG. 26 is a schematic diagram indicating the operation of a beamdeflecting unit using a conventional rotating polygon mirror;

FIG. 27 is an optical side showing an optical path of a conventionallaser beam scanning optical system;

FIG. 28 is a schematic diagram showing a conventional edge emitting typesemiconductor laser;

FIG. 29 is a schematic diagram showing a laser scanning optical systemof a fourth embodiment of the image forming apparatus in accordance withthe present invention;

FIG. 30 is a side view showing the image forming apparatus;

FIG. 31 is a sectional view showing an optical resonator of a surfacelight emitting type semiconductor laser;

FIG. 32 comprises schematic diagrams showing the relations of scanninglines and spot positions;

FIG. 33 is an optical side view showing an optical path of aconventional laser beam scanning optical system;

FIG. 34 is an optical side view showing an optical path of aconventional multi-beam scanning system;

FIG. 35 is a schematic diagram showing a conventional edge emitting typesemiconductor laser;

FIG. 36 is an optical side of an optical path including a conventionaltilt angle compensation lens;

FIG. 37 is a schematic diagram showing a laser scanning optical systemof a fifth embodiment of the image forming apparatus in accordance withthe present invention;

FIG. 38 is a side view showing the image forming apparatus;

FIG. 39 comprises an optical side view and a diagrams showing theconstruction of portions adjacent to a light source of the scanningoptical system;

FIG. 40 is an optical side view showing the construction of portionsadjacent to a light source of a scanning optical system of anotherembodiment;

FIG. 41 is an optical side view showing the construction of portionsadjacent to a light source of a conventional scanning optical system;

FIG. 42 is an optical side view showing a laser scanning optical systemof a sixth embodiment of the image forming apparatus in accordance withthe present invention;

FIG. 43 is an optical side view showing the construction of portionsadjacent to a light source of a conventional optical system;

FIG. 44 is an optical side view describing that a beam is vignetted by acollimator lens;

FIG. 45 is an optical side view showing a beam which passes through anaperture stop disposed at a focal point of a collimator lens;

FIG. 46 is an optical side view showing a beam which passes through anaperture stop disposed at other than a focal point of a collimator lens;

FIG. 47 comprises charts showing distributions of beam sectionalintensity in the case where a beam is vignetted;

FIG. 48 is a side view showing the image forming apparatus; and

FIG. 49 is an optical side view showing the construction of portionsadjacent to a light source of a scanning optical system.

BEST MODES FOR CARRYING OUT THE INVENTION

Section 1 First Embodiment of Image Forming Apparatus

1-1 Comparison with Related Art

Before describing a first embodiment of the present invention, therelated art thereof will be first described as conducive to a fullunderstanding of the conception thereof.

FIG. 7 shows a conventional semiconductor laser array for use in animage forming apparatus. As shown in the figure, in a semiconductorlaser array 1 which emits a laser beam, the spread angle of a laser beamon a plane which includes the optical axis thereof and which is inparallel with the contact surface largely differs from that on a planewhich contains the optical axis and which is perpendicular to thecontact surface. In the figure, the spread angle θp of a conventionallaser diode on the plane in parallel with the contact surface isapproximately 10 degrees in full width at half maximum. On the otherhand, the spread angle θt on the plane perpendicular to the contactsurface is as large as 30 degrees in full width at half maximum due tothe influence of diffraction. In addition, it is difficult to freely setthe values of the spread angles θt and θp and the ratio thereof (inother words, the ratio of the longer diameter and the shorter diameterof the ellipse). As a result, the position of the beam waist on theparallel plane differs from that of the vertical plane by δ. The valueof δ is generally referred to as an astigmatic difference.

Due to this astigmatic difference, the beam which leaves a collimatorlens is not in parallel with either the scanning plane or the directionperpendicular thereto or both of them. Thus, the beam cannot beprecisely spotted on a image bearing member, but it has an aberration.Since the focal length of an image forming lens for use in aconventional laser printer is long and the spot diameter thereof islarge, this aberration does not lead to a considerable problem. However,as the need for high resolution printers arises in recent years,aberration is becoming a critical problem to solve. As one of thetechniques for solving this problem, a beam shaping optical system whichcomprises a so-called anamorphic lens set where the power on thevertical plane differs from that on the horizontal plane is used tocompensate for the astigmatic difference. However, such a beam shapingoptical system is liable to raise the cost of the final product and toincrease the size thereof. In addition, this system cannot be easilyapplied to an application which scans a plurality of laser beams.

Moreover, since a laser beam is emitted from an end face of thesemiconductor laser array 1, the light emitting portion of the laserbeam should be necessarily disposed one-dimensionally in line. To obtaina large number of laser beams, since the laser beams are arranged inline, the effective diameter of the optical system becomes large.

Further, because the size of the spread angle is large, the focal lengthof the collimator lens which collimates the beams becomes as small asseveral millimeters. Even if the distance between the semiconductorlaser array and the collimator lens slightly varies for example by theorder of several 10 μm, the resultant rays (parallel beams) are notcollimated rays. Thus, the diameter of the beam introduced into an imageforming optical system deviates and the size of the image spot on theimage bearing member varies. Therefore, the allowable ranges of thesemiconductor laser and the collimator lens become very small. As aresult, the producibility is low. In addition, the position of thecollimator lens which has been precisely adjusted is moved due totemperature change of the adjacent portions of the optical system andaging deformation due to aging of the constructional parts. Thus, thediameter of the image spot varies and thereby the image qualitydeteriorates.

Furthermore, when a plurality of laser beams having parallel opticalaxes enter the collimator lens, the optical axes are spread with largeangles. Now, for simplicity, a laser scanning optical system using twolaser beams, a convex collimator lens, and a convex image forming lenswill be considered. FIG. 8 is an optical side view showing the opticalpath of this optical system. The two laser beams spaced apart by d andemitted from the semiconductor laser array 1 are collimated by thecollimator lens 2 of a focal length fc. Since the semiconductor laserarray 1 is disposed at an object side focal point of the collimator lens2, the two laser beams are intersected at an image side focal point F.To form images of the two laser beams which are nearly parallel on animage plane 11, the image forming lens 4 of the focal length fi isdisposed in such a way that the object side focal point thereof matchesthe image side focal point F of the collimator lens 2. Since the mirrorsurface of the deflecting unit does not have an optical power, it isomitted in the figure. When a spot 6 of 100 μm is imaged on the imageplane 11, if fi is 200 mm, the diameter Wc of the beam introduced intothe image forming lens, that is, the collimate diameter, isapproximately 2 mm. The spot diameter or the beam diameter is a diameterwhere the intensity of the cross section of a beam is the power of thepeak intensity×(1/e²). The distribution of this intensity accords withthe Gaussian distribution. To obtain the above-mentioned beam diameterof 2 mm, the focal length fc of the collimator lens 2 should beapproximately 3 mm. As shown in FIG. 8, the distance d′ between thesespots is obtained by multiplying the ratio of fc and fi by d. In theconventional semiconductor laser arrays, it is difficult to set thedistance between each light emitting portion to 100 μm or below due tomutual interference thereof. Thus, in this example, the spot distance d′on the image plane can be expressed as follows. $\begin{matrix}{d^{\prime} = {{\frac{fi}{fc} \times d} = {{\frac{200}{3} \times 0.1} = {6.6\quad ({mm})}}}} & (1)\end{matrix}$

In addition, when a so-called tilt angle compensation optical systemwhich compensates for the difference of the tilt angle of each facet ofa rotating polygon mirror is used, the angle made of the optical axis ofeach laser beam sometimes becomes large depending on the relativedistance between each lens and the collimator lens. To overcome thisproblem, various countermeasures such as adding another lens areconsidered. For example, as disclosed in Japanese Patent Laid-OpenPublication Serial No. SHO 58-211735, a construction for compensatingfor the mutual angle of the optical axis of each laser beam by using aprism has been proposed. However, this construction leads to acomplicating the optical system. Thus, the cost of the final productrises and the adjustment of the optical system becomes difficult. InFIG. 8, for simplicity, the tilt angle compensating lens is omitted.

Next, referring to FIG. 9, the relation of spot positions and scanninglines on a conventional image bearing member is shown. In this example,there are four spots. In other words, images are formed with four laserbeams. As described above, since the laser scanning optical system is anenlarging optical system, the distance of each adjacent spot on thesemiconductor laser array is enlarged on the image bearing member to d′as shown in the figure. Normally, the distance of each adjacent spot onthe image bearing member is much larger than the distance P of eachadjacent scanning line 9. For example, when the resolution is 300 dpi(that is, the number of dots per inch (=25.4 mm)), althoughP=25.4/300=84.7 μm, the distance of each adjacent spot becomes as largeas 6.7 mm. Thus, the angle α made of a line 12 connecting the center ofthe spot 6 and each scanning line 9 becomes very small as follows.$\begin{matrix}{\alpha = {{\sin^{- 1}\frac{P}{d^{\prime}}} = {0.72\quad ( \deg )}}} & (2)\end{matrix}$

In addition, the line which connects the light emitting portions on thesemiconductor laser array 1 (that is, the edge of the contact surface)should have an angle of α to the scanning plane. As the value of αbecomes small, very fine adjustment is required.

Generally, a laser beam emitted from a semiconductor laser is linearlypolarized. The direction of the plane of polarization of the laser beamis determined by the inclination of the contact surface of thesemiconductor laser array. However, the reflectance on a reflectingsurface depends on the incident angle to the mirror surface. Inaddition, the reflectance of P polarized light differs from that of Spolarized light. FIG. 10 shows respective reflectances Rp and Rs of Ppolarized light and S polarized light on a metallic mirror. As thepolygon mirror rotates, the incident angle of the beam relative to themirror surface varies. Thus, as shown in the figure, the amount of lightof the laser beam represented as a composition of P polarized light andS polarized light also varies. In particular, when the deflection angleof the polygon mirror is large, the amount of light of the laser beamremarkably varies. To overcome this problem, as disclosed in JapanesePatent Laid-Open Publication Serial No. SHO 58-42025, a technique forinclining the plane of polarization about the rotating axis of therotating polygon mirror by 45 degrees has been proposed. However, asdescribed above, in the edge emitting type semiconductor laser array 1,due to the restriction of the distance of each adjacent scanning line,the inclined angle is also restricted. Thus, this technique cannot beused. In this case, the plane of polarization should be rotated with a¼λ plate or the like.

In addition, each laser beam emitted from the semiconductor laser array1 enters the same collimator lens 2. At this point, as shown in FIG. 8,the collimate diameter Wc of each laser beam is determined by the spreadangle θ thereof and the distance fc between the semiconductor laserarray 1 and the collimator lens 2. However, since this distance fc ofeach adjacent laser beam is the same, the collimate diameter Wc isdetermined by only the spread angle θ of the laser beam. Nevertheless,in the conventional edge emitting type semiconductor laser array 1, thespread angle varies depending on each light emitting portion, so doesthe collimate diameter Wc of each laser beam. As a result, the size ofeach spot where each collimated laser beam is imaged also varies. In aconventional laser scanning optical system which uses only one (single)laser beam, as shown in FIG. 11, with aperture stop 13 disposed eitherin the front or behind the collimator lens 2, the beam is shaped so asto adjust the collimator diameter Wc. However, as shown in FIG. 8, whena plurality of laser beams in a bundle are used, the aperture stop canbe disposed at only the a focal point of the collimator lens.

Generally, in a conventional semiconductor laser array, unless thecurrent which flows in an optical resonator exceeds a predeterminedvalue, laser oscillation does not take place. This current value isreferred to as the threshold current value. In the conventionalsemiconductor laser array, the threshold current value is as the high asseveral 10 mA. The heat generated by the current adversely affects thecharacteristics of the laser beams, such as shifting the oscillationwavelength. Thus, the heat generated by the semiconductor laser array isa problem to be solved. In particular, a semiconductor laser array whichemits a plurality of laser beams has a number of heat sources equal tothe number of light emitting parts. This heat has been an obstacle inintegrally constructing a large number of light emitting parts.

1-2 Organization of the Present Invention

Next, an embodiment of the present invention will be described. FIG. 2is a schematic diagram showing the overall structure of an image formingapparatus in accordance with the present invention. The process forobtaining a print result on an image transfer material 51 accords withthe so-called electrophotographic process. As a image bearing member 5of an electrophotographic printer using a semiconductor laser as a lightsource, an organic photoconductor (OPC) with an increased sensitivity ina longer wavelength region is widely used. This image bearing member 5is charged to a predetermined surface potential by a charger 52.Thereafter, a laser beam scanning unit 53 performs a light writingprocess, that is, a light exposing process. In accordance with imageinformation from the laser beam scanning unit 53, a plurality of laserbeams 54 whose light intensities are individually modulated are scannedon image bearing member 5 in the axial direction thereof, therebygenerating electric charges which neutralize the surface potential onlyfor the exposed portion. Thus, the absolute value of the surfacepotential of this portion becomes low. As a result, on the image bearingmember 5, a distribution of surface potential in accordance with theimage, that is, a static latent image is formed. A developing unit 55selectively adheres a developing agent in accordance with the surfacepotentials to the image bearing member 5. Thus, the static latent imageis developed. This developing agent is transferred to a transfermaterial 51 (normally, a paper) by a transferring unit 56. Thedeveloping agent on the transfer material 51 is fixed with a thermalpressure by a fixing unit 57. Thereafter, the transfer material 51 isunloaded from the apparatus.

FIG. 1 is a schematic diagram showing the organization of a laser beamscanning unit 53 for use in the image forming apparatus of the presentinvention. In the laser beam scanning unit 53 shown in FIG. 2, the laserbeams 54 are folded back and downwardly emitted. However, in the figure,the illustration of the laser beams is simplified.

In FIG. 1, a semiconductor laser array 21 comprises a plurality of lightemitting portions 21 a which are disposed two-dimensionally on a devicesubstrate 22 (see FIG. 3). Laser beams emitted from the light emittingportions 21 are collimated to laser beams with a predetermined beamdiameter by a collimator lens 2. The lighting and the amount of light ofeach of the light emitting portions 21 a is discretely controlled by acontrol unit 60. The laser beams are introduced into one facet of arotating polygon mirror 3. As the polygon mirror 3 rotates, these laserbeams are deflected. The laser beams which pass through an image forminglens 4 are imaged at spots 6 on the image bearing member 5. In FIG. 1,for simplicity, a tilt angle compensation lens is omitted.

As the semiconductor laser array 21 having such characteristics, it ispreferable to use a so-called surface light emitting type semiconductorlaser array. It is more preferable to use a surface light emitting typesemiconductor laser array with light emitting portions in which a groupII-VI compound semiconductor is embedded. FIG. 3 is a sectional viewshowing one of the light emitting portions 21 a of the surface lightemitting type semiconductor laser array 21. In the figure, each of thelight emitting portions two-dimensionally disposed on the devicesubstrate 22 is provided with one optical resonator.

As shown in the figure, on the Ga—As device substrate 22 a,semiconductor laminate reflecting layer 23 is formed. The reflectinglayer 23 is composed of several tens of layers of two types of Al—Ga—Ascompounds. On the reflecting layer 23 a, clad layer 24, an active layer25, a clad layer 26, and a contact layer 27 are disposed, each of whichis composed of Al—Ga—As compounds. On the contact layer 27, a SiO₂dielectric laminate reflecting layer 28 is formed. On the entire rearsurface of the Ga—As substrate, a window-shaped electrode 29 is formed.In addition, in the periphery of the dielectric laminate reflectinglayer 28, a window-shaped electrode 30 is formed. Thus, all the partsformed on the Ga—As substrate constitute an optical resonator.

A light beam which is generated on the active layer 25 reciprocativelytravels between the upper reflecting layer 27 and the lower reflectinglayer 23 in the direction perpendicular to the surface of the devicesubstrate 22. Thus, the light beam is oscillated. As a result, theoptical axis of the laser beam 31 is substantially perpendicular to thesurface of the device substrate 22. In the periphery of the opticalresonator, a group II-VI compound semiconductor is embedded as anembedded layer 32. As the group II-VI compound semiconductor, it ispreferable to use a group II-VI compound which contains two, three, orfour elements selected from both group II elements Zn, Cd, and Hg andgroup VI elements O, S, Se, and Te. In addition, it is preferable tomatch the lattice constant of the compound with that of thesemiconductor layers composed of the clad layer 24, the active layer 25,and the clad layer 26. Since the electric resistance of the group II-VIcompound semiconductor is very high, the current is effectively closedin the optical resonator. In addition, since the refractive index of theembedded layer 32 differs from that of the Al—Ga—As semiconductor layer,the beam which travels in the optical resonator in the exactly just orsubstantially perpendicular to the surface of the device substrate 22 istotally reflected at the interface with the embedded layer 32. Thus, thebeam is efficiently closed in the optical resonator.

Therefore, when such a semiconductor laser array 21 is used, the laseroscillation starts with a very small amount of current in comparisonwith the conventional laser array. In other words, the threshold valueof the semiconductor laser array 21 is lower than that of theconventional one. Thus, even if a plurality of light emitting portions21 a are disposed on the single device substrate 22, the amount of lostheat is small and thereby high optical power or a large number of lightemitting portions 21 a can be obtained.

In addition, since the sectional area (near field pattern) of the laserbeam emitting portions (light emitting portions) 21 a of the surfacelight emitting type semiconductor laser array 21 is larger than that ofthe conventional edge emitting type semiconductor laser array, thespread angles of the laser beams are small. Although the value of thespread angle is determined by the area of the light emitting window, thearea can be precisely controlled by an etching process or the like.Thus, the spread angle can be kept constant. Moreover, the ratio of thelength and width of the spread angle of a laser beam, namely, the ratioof the longer diameter and shorter diameter of a beam whose crosssection is in an elliptic shape can be freely set with the shape of thelight emitting window. For example, when the shape of the window is aperfect circle, a laser beam with a circular cross section or anisotropic spread angle can be obtained. Thus, the astigmatic differenceon the cross section in the direction of the optical axis of a beam issmall.

However, in a conventional laser beam printer, the image spot of a laserbeam on the image bearing member is sometimes in an elliptic shape wherethe scanning direction of the laser beam accords with the minor axisthereof. Since the spot moves in the scanning direction for the lightingtime of the laser beam and thereby the image is expanded, the imageshould be corrected. Thus, the shape of the cross section of the laserbeam which enters the image forming optical system is preferablyelliptic. As described above, in the surface light emitting typesemiconductor laser array 21, since the elliptic ratio of each emittedlaser beam can be freely controlled, the laser beam with a cross sectionwhose major axis accords with the scanning plane and whose ratio of themajor axis and the minor axis is appropriate can be introduced into theimage forming optical system.

Moreover, as described above, in the surface light emitting typesemiconductor laser array 21, the spread angle of the laser beams fromrespective light emitting portions 21 a can be made uniform. Thus, thediameter of each laser beam introduced into the collimator lens or theimage forming optical system can be kept nearly constant. As a result,the size of each image spot on the image bearing member can be keptconstant.

As the sectional area of the optical resonator increases on the surfaceof the device substrate 22 of the surface light emitting typesemiconductor laser array 21 becomes large, laser oscillations of highorder modes along with zero-th mode start. The distribution of theamount of light of the image spot has several peaks. Thus, thissituation is not suitable for forming a static latent image on the imagebearing member 5. To prevent this situation, a plurality of smalloptical resonators are closely disposed, and the phases of the laserbeams oscillated are synchronized. As a result, the light emittingportions 21 a which are large in size and oscillated in the zero-th modecan be obtained.

FIG. 4 shows a partial sectional view of a light emitting portion 21 aof the phase-lock surface-light-emitting type semiconductor laser array21. In this semiconductor laser array 21, a plurality of opticalresonators are disposed at very short intervals. The bottom of theembedded layer 32 does not reach the active layer 25. Thus, rays whichleak out from the adjacent optical resonators adversely affect eachother through the clad layer 26 below the embedded layer 32. Thus, laserbeams are oscillated with the same phase. As a result, the plurality ofadjacent optical resonators operate as a single optical resonator. Sincethe wave faces of beams emitted from the optical resonators are matched,these optical resonators operate as a surface laser emitting source.Thus, the apparent area of the light emitting portions becomes large.The spread angles of laser beams are as small as 2 degrees or less infull width at half maximum.

As described above, in the phase-lock surface-light-emitting typesemiconductor laser array 21, the spread angles of the laser beams aresmall in comparison with the conventional semiconductor laser. Withrespect to this characteristic, the relation between the embodiment ofthe present invention and the embodiment of the related art will bedescribed. When the spread angle of a laser beam is 2 degrees in fullwidth at half maximum and the laser beam is introduced into an imageforming optical system with a beam diameter of 2 mm, the focal length fcof the collimator lens becomes approximately 35 mm. Since the focallength fc of the collimator lens 2 can become long, the adjustmentallowance of the distance of the collimator lens 2 to the semiconductorlaser array 21 is increased. In addition, when the distance of eachadjacent laser beam emitted from the semiconductor laser array 21 is d,the distance d′ of each adjacent image spot can be given by thefollowing formula. $\begin{matrix}{d^{\prime} = {{\frac{fi}{fc} \times d} = {{\frac{200}{35} \times 0.1} = {0.57\quad ({mm})}}}} & (3)\end{matrix}$

When the distance of each adjacent scanning line is 84.7 μm which is thesame as that of the related art, the angle α made of each spot and thescanning plane is given by the following formula. $\begin{matrix}{\alpha = {{\sin^{- 1}\frac{P}{d^{\prime}}} = {8.55\quad ( \deg )}}} & (4)\end{matrix}$

Thus, the angle α of the semiconductor laser array 21 is much largerthan that of the conventional semiconductor laser array (see Formula(3)). Therefore, the mounting adjustment in the direction of the opticalaxis of the semiconductor laser array 21 can be easily performed. Inaddition, depending on the machining tolerance of each part, the partcan be mounted without necessity of the adjustment of the angle α.Moreover, since the image spots and the laser beams are closelydisposed, the effective diameter of the optical system becomes small asshown in the figure.

Further, by excessively decreasing the spread angle of each laser beam,after the laser beam travels from the semiconductor laser array 21 tothe rotating polygon mirror 3 to the image forming lens 4, the laserbeam is not largely spread. Thus, even on the incident plane of theimage forming lens 4, the size of the laser beam can be decreased enoughto obtain a predetermined diameter of an image spot. In other words, acollimator lens which collimates a laser beam with a predeterminedcollimate diameter is not required in contrast to the conventional laserscanning optical system. However, according to the deflection angle ofthe rotating polygon mirror 3, the length of the optical path varies, sodoes the size of the laser beam introduced into the image forming lens4. Thus, there should be provided an optical system which corrects thesize of the laser beam. Nevertheless, the image forming lens 4 can beeasily provided with such an optical function. Thus, the number ofconstituent parts of the overall optical system can be decreased.

Moreover, in the surface light emitting type semiconductor laser array21, provided that beams of the light emitting portions 21 a do notinterfere each other, these light emitting portions 21 a can be disposedat any positions of the device substrate 22. Thus, the light emittingportions 21 a can be two-dimensionally disposed on the device substrate22. Now, an exposure system which scans a image bearing member 5 withfour laser beams like the optical system of the embodiment of the priorart as shown in FIG. 8 will be considered. When four laser beams aredisposed as shown in FIG. 5(a), the mutual angles and distances of eachthe adjacent laser beams can be decreased in comparison with the casewhere four laser beams are disposed in line as shown in FIG. 5(b). As aresult, the size of the optical system can be accordingly decreased.

In the above example, the case where four laser beams are used wasconsidered. When the number of laser beams is further increased, thelight emitting portions 21 a can be freely disposed on the semiconductorlaser array 21 so that the spots 6 are disposed at the closest positionson the image bearing member 5. As a result, more significant effects canbe obtained than by the above described arrangements. In FIG. 5(c), anexample of the arrangement of image spots 6 to scanning lines 9 in thecase of eight laser beams is shown.

The relative positions of image spots are not always similar to thepositions of the light emitting portions 21 a on the semiconductor laserarray 21. For example, like the above-mentioned optical system forcompensating tilt angle of the rotating polygon mirror 3, if an opticalelement wherein the optical characteristics in the scanning directiondiffer from those in the perpendicular direction thereof is disposedmidway in the optical path of laser beams, the angle and distance ofeach adjacent laser beam in the scanning direction sometimes differ fromthose in the perpendicular direction thereof. However, in such asituation, according to the conventional edge emitting laser array, thelight emitting portions disposed in line at most allow image spots inline to be disposed on the image bearing member. In contrast, accordingto the present invention, the above-mentioned effect where the lightemitting portions 21 a are disposed two-dimensionally on the devicesubstrate 22 can be likewise accomplished.

In the surface light emitting type semiconductor laser array 21,generally the emitted laser beams are linearly polarized. The directionof the polarization depends on the shape of the plane of the opticalresonators on the device substrate 22. The plane of polarizationsubstantially accords with the longitudinal direction of the shape ofthe plane of each optical resonator. For example, when the opticalresonator is in an elliptic shape, the direction of the major axisthereof becomes the plane of polarization. As described above, in thephase synchronizing type semiconductor laser array 21, one lightemitting portion is composed of a plurality of phase synchronizingoptical resonators. The number of optical resonators is for examplefour. At this point, the shape of the cross section of the laser beamsemitted is a composite shape of laser beams emitted from all the opticalresonators. Thus, in accordance with the arrangement of the opticalresonators, the shape of the cross section of the laser beams emittedcan be freely set. In this case, the orientation of the plane ofpolarization depends on the shape of the plane of each opticalresonator. Thus, in the case where laser beams in a composite ellipticshape are obtained, the major axis and the direction of the plane ofpolarization can be independently set.

FIG. 6(a) schematically shows this situation. This figure is a plan viewof one light emitting portion 42 on a semiconductor laser array, thelight emitting portion 42 being seen from the beam emitting side. Asshown in FIG. 6(a), one light emitting portion 42 is composed of fouroptical resonators 41 which phase-synchronously oscillate. The plane ofpolarization 43 of a laser beam emitted from each optical resonator 41is inclined by 45 degrees as shown in FIG. 6(a). The major axis of thecomposite elliptic laser beam accords with the vertical direction of thelight emitting portion. In addition, as shown in FIG. 6(b), when theplanes of polarization 43 of the laser beams emitted from the opticalresonators 41 are oriented at different angles, the composite laser beamnearly becomes circularly polarized light.

As described above, in the conventional laser beam printer, an imagespot 6 on the image bearing member is often in an elliptic shape wherethe minor axis thereof accords with the scanning direction. When thesemiconductor laser array 21 is disposed in such a way that theorientation of the plane of polarization of a laser beam from eachoptical resonator is inclined to the direction of the major axis of theelliptic cross section of the composite laser beam by 45 degrees and themajor axis of the composite beam is matched with the scanning direction,the plane of polarization of the laser beam from each optical resonatoris inclined to the beam scanning plane by 45 degrees. As a result, theplane of polarization is inclined to the rotating axis of the rotatingpolygon mirror 3 by 45 degrees. Thus, as shown in FIG. 10, thedifference of reflectances of laser beams according to incident anglesthereof to the rotating polygon mirror becomes small. This situationalso applies to a laser beam with a cross section of an elliptic shape,the beam being nearly circularly polarized light. In some opticalsystems, the minor axis of a laser beam emitted from the semiconductorlaser array 21 is matched with the scanning direction. In thissituation, the same effects can be obtained.

It should be appreciated that the above-mentioned embodiment is anexample of the present invention. For example, by using a Galvano mirroror a hologram disk instead of the rotating polygon mirror 3, the sameeffects can be obtained. In addition, regardless of whether thecollimator lens, the tilt angle compensation lens, and/or the imageforming lens is used or not, the same effects can be obtained. Moreover,even if the construction and/or relative positions of these lenses arechanged, the same effects of the present invention can be likewiseachieved.

Furthermore, the image forming apparatus of the present invention can beused for facsimile machines, display units, and so forth as well asprinting units such as printers and copy machine.

1-3 Effects

As described above, in the image forming apparatus of the presentinvention, since the exposing technique using the semiconductor laserarray which scans the image bearing member with a plurality of laserbeams, a high-speed and high-resolution scanning unit can beaccomplished with a low scanning frequency and a short length of theoptical path. Thus, the size and cost of the apparatus can be reduced.

In addition, by using a surface light emitting type semiconductor laserarray for the above-mentioned semiconductor laser array, the followingresults can be obtained.

(1) Since the spread angle of each leaser beam is small and the distancebetween the collimator lens and the semiconductor laser array is large,the adjustment allowance in the direction of the optical axis of thecollimator lens is increased and thereby the producibility of theapparatus is improved. Moreover, without influences of ageddeterioration and temperature fluctuation in operation, an image can beexposed with a predetermined spot diameter. As a result, the imagequality is improved.

(2) When the light emitting portions are disposed in an array shape, thespread angle of a beam from each light emitting portion less deviates,so does the diameter of each image spot. In addition, the angle betweeneach adjacent beam and the distance of each adjacent image spot can bedecreased. As a result, the construction of the optical system can besimplified and the effective areas of each lens and deflecting unit canbe decreased. Therefore, this feature contributes to reducing the costof the apparatus.

(3) Since the light emitting portions of the surface light emitting typesemiconductor laser array can be disposed two-dimensionally, theeffective areas of each lens and deflecting unit can be further reduced.

(4) Since the distance of each adjacent image spot is not larger thanthat of each adjacent scanning line, the allowance of the mounting angleof the optical axis of the semiconductor laser array becomes large.Thus, this mounting angle can be easily adjusted. The deviation of thedistance of each adjacent scanning line becomes small. As a result, theimage quality can be improved.

(5) Since the astigmatic difference of the surface light emitting typesemiconductor laser array is low as a characteristic thereof, theelliptic shape of the cross section of each beam (ratio of major axisand minor axis) can be freely set. Thus, without necessity of an opticalsystem which compensates the astigmatic difference, each beam can beprecisely shaped.

(6) Since a group II-VI compound semiconductor is used for the surfacelight emitting type semiconductor laser array as an embedded layer, thelaser oscillation can be accomplished with a low threshold currentvalue. Thus, the bad influence of heat generated by the device againstthe laser characteristics can be reduced. As a result, a large number oflight emitting portions can be integrally disposed.

(7) When a phase-lock surface-light-emitting type semiconductor laserarray with a plurality of optical resonators which emit phase-lockedlaser beams is used as the surface light emitting type semiconductorlaser, the spread angle of each laser beam can be further decreased. Insome cases, the collimator lens can be omitted. Thus, the constructionof the optical system can be further simplified.

(8) Since one light emitting portion can be composed of a plurality ofoptical resonators whose plane of polarization can be freely set, when acomposite laser beam with an elliptic cross section is used, thedirection of the plane of polarization of each laser beam can be setfreely and independent from the direction of the major axis of the crosssection of the composite laser beam. As a result, the fluctuation of theamount of light according to the position in the scanning direction ofthe laser beam due to the difference of incident angle of the laser beamto the polygon mirror can be minimized.

Section 2 Second Embodiment of Image Forming Apparatus

2-1 Comparison with Related Art

Before describing a second embodiment of the present invention, so as toeasily understand the conception thereof, the related art thereof willbe described.

FIG. 17 is a sectional view showing an optical path of a conventionalimage forming apparatus. The figure shows a horizontal section of theoptical path which is perpendicular to a scanning plane of a imagebearing member of the image forming apparatus and which contains anoptical axis of a laser beam. In the figure, the optical axis is foldedback at a facet 108 of a rotating polygon mirror 8 of the image formingapparatus. In the 30 figure, a laser beam radiated from a semiconductorlaser 101 is emitted at a spread angle of θ. This beam is shaped to anearly collimated beam by a collimator lens 102 of a focal length fc.Each beam is collected at the facet 108 of the rotating polygon mirrorby a tilt angle compensation lens 107. The beam deflected by therotating polygon mirror is collimated by a second tilt anglecompensation lens 107′. Thereafter, the beam is imaged as a spot 106 onthe image bearing member by an image forming lens 104 of a focal lengthfi. On a plane in parallel with the scanning plane, since the tilt anglecompensation lenses 107 and 107′ do not have optical powers. Thus on theplane, each beam is kept parallel. In other words, on the facet 108 ofthe rotating polygon mirror, each beam is imaged as a line image.

However, as shown with a conceptional schematic diagram of FIG. 18, ineach laser beam from the semiconductor laser 101, the spread angle of alaser beam on a plane which includes the optical axis thereof and whichis in parallel with the contact surface largely differs from that on aplane which contains the optical axis and which is perpendicular to thecontact surface. In the figure, the spread angle θt of a conventionallaser diode on the plane in parallel with the contact surface isapproximately 10 degrees in full width at half maximum. On the otherhand, the spread angle θt on the plane perpendicular to the contactsurface is as large as 30 degrees in full width at half maximum due toan influence of diffraction. In addition, it is difficult to freely setthe values of the spread angles θt and θp and the ratio thereof (inother words, the ratio of the longer diameter and the shorter diameterof the ellipse). As a result, the position of the beam waist on theparallel plane differs from that of the vertical plane by d. The valueof d is generally referred to as an astigmatic difference.

Due to this astigmatic difference, the beam which leaves a collimatorlens is not in parallel with either the scanning plane or the directionperpendicular thereof or both of them. Thus, the beam cannot beprecisely spotted on a image bearing member, but it has an aberration.

Since the focal length of an image forming lens for use in aconventional image forming apparatus is long and the spot diameterthereof is large, this aberration does not lead to a considerableproblem. However, as the need of high resolution printers stronglyarises in recent years, the aberration is becoming a critical problem tosolve. As one of the techniques for solving this problem, a beam shapingoptical system which comprises a so-called anamorphic lens set where thepower on the vertical plane differs from that on the horizontal plane isused to compensate the astigmatic difference. However, such a beamshaping optical system is liable to raise the cost of the final productand to increase the size thereof.

Next, a problem which arises due to a large spread angle of each beamwill be described with reference to FIG. 17. When a spot 106 of 100 μmis imaged on the image plane 111, if fi is 200 mm, the diameter Wc ofthe beam entered into the image forming lens, that is, the collimatediameter, is approximately 2 mm. The spot diameter or the beam diameteris a diameter where the intensity of the cross section of a beam is thepower of the peak intensity×(1/e²). The tilt angle compensation lenses107 and 107′ are symmetrical with respect to the facet 108 of therotating polygon mirror 108. The diameter of a beam emitted from thelens 107′ is the same as the diameter of a beam entered into the lens107. To obtain this diameter of a beam, the focal length fc of thecollimator lens 102 should be approximately 3 mm.

Since the focal length of the collimator lens 102 is short, to obtain abeam which is completely collimated, the error of position in thedirection of the optical axis of the collimator lens should be as smallas possible. In addition, the spread angles θt and θp of the beamsometimes deviate due to problems in semiconductor's producingprocesses. As a result, the diameters of beams which are collimateddeviate. Thus, it is necessary to dispose a slit or an aperture stopbehind the collimator lens 102. In addition, even if the collimator lenshas been precisely adjusted in the initial stage, the position of thecollimator lens 102 deviates due to temperature rise of the peripheralportions of the optical system in operation and aged deterioration ofconstructional parts. Thus, the diameters of image spots deviate andthereby the image quality deteriorates.

Generally, a laser beam emitted from a semiconductor laser is linearlypolarized. The direction of the plane of polarization of the laser beamjust depends on the inclination of the contact surface of thesemiconductor laser. However, the reflectance on a reflecting surfacedepends on the incident angle to the mirror surface. In addition, thereflectance of P polarized light differs from that of S polarized light.FIG. 19 shows respective reflectances Rp and Rs of P polarized light andS polarized light on a metallic mirror.

As the rotating polygon mirror rotates, the incident angle of the beamto the mirror surface varies. Thus, as shown in the figure, the amountof light of the laser beam represented as a composition of P polarizedlight and S polarized light also varies. In particular, when thedeflection angle of the rotating polygon mirror is large, the amount oflight of the laser beam remarkably varies. To prevent this problem, asdisclosed in Japanese Patent Laid-Open Publication Serial No. SHO58-42025, a technique for inclining the plane of polarization about therotating axis of the rotating polygon mirror by 45 degrees has beenproposed. However, in this technique, the direction of the major axis ofthe elliptic section of each beam is fixed. Thus, this technique cannotbe used. Alternatively, the plane of polarization should be rotated witha ¼λ plate or the like.

Generally, in a conventional semiconductor laser, unless the currentwhich flows in an optical resonator exceeds a predetermined value, thelaser oscillation does not take place. This current value is referred toas a threshold current value. In the conventional semiconductor laser,the threshold current value is as high as several 10 mA. The heatgenerated by the current adversely affects the characteristics of thelaser beams, such as shifting the oscillation wavelength. Thus, the heatgenerated by the semiconductor laser is a problem to solve.

2-2 Construction of the Present Invention

Next, an embodiment of the present invention will be described. FIG. 13is a schematic diagram showing the overall construction of an imageforming apparatus in accordance with the present invention. The processfor obtaining a print result on an image transfer material 151 accordswith a so-called electrophotographic process. As aphotoconductivelycoated drum 105 of an electrophotographic printer usingasemiconductor laser as a light source, an organicphotoconductor (OPC)with an increased sensitivity in a longerwavelength region is widelyused. This image bearing member 105 is charged to a predeterminedsurface potential by a charger 152. Thereafter, a laser beam scanningunit 153 performs a light writing process, that is, a light exposingprocess. In accordance with image information from the laser beamscanning unit 153, a plurality of laser beams 154 whose lightintensities are individually modulated are scanned on image bearingmember 105 in the axial direction thereof, thereby generating electriccharges which neutralize the surface potential only for the exposedportion. Thus, the absolute value of the surface potential of thisportion becomes low. As a result, on the image bearing member 105, adistribution of surface potentials in accordance with the image, thatis, a static latent image is formed. A developing unit 155 selectivelyadheres a developing agent in accordance with the surface potentials tothe image bearing member 105. Thus, the static latent image isdeveloped. This developing agent is transferred to a transfer material151 (normally, a paper) by a transferring unit 156. The developing agenton the transfer material 151 is fixed with a thermal pressure by afixing unit 157. Thereafter, the transfer material 151 is unloaded fromthe apparatus.

Next, with reference to FIG. 12, a laser beam scanning unit will bedescribed. In the laser beam scanning unit 153 shown in FIG. 13, thelaser beam 154 is folded back and downwardly emitted. In FIG. 12, theoptical path of a laser beam is simplified. In the figure, in asemiconductor laser 121, a laser beam is emitted from a light emittingportion 121 a in the direction perpendicular to a contact surface. Thelighting and the amount of light of the light emitting portion 121 a arecontrolled by a control unit 160. The beam is collimated to a laser beamof a predetermined diameter by a collimator lens 102. The laser beam isentered into a facet of a rotating polygon mirror 103. As the rotatingpolygon mirror 103 rotates, the beam which passes through an imageforming lens 104 is imaged at a spot on a image bearing member.

As a semiconductor laser having such characteristics, it is preferableto use a so-called surface light emitting type semiconductor laser. Itis more preferable to use a surface light emitting type semiconductorlaser with a light emitting portion 121 a in which a group II-VIcompound semiconductor is embedded. FIG. 14 is a sectional view showingthe light emitting portion 121 a of the surface light emitting typesemiconductor laser. In the figure, one optical resonator constructs onelight emitting portion. As shown in FIG. 14, on the Ga—As devicesubstrate 122, a clad layer 124, an active layer 125, a clad layer 126,and a contact layer 127 are disposed, each of which is composed ofseveral ten layers of two types of Al—Ga—As compounds. On the contactlayer 127, a SiO2 dielectric laminate reflecting layer 128 is formed. Onthe entire rear surface of the Ga—As substrate 122, a window-shapedelectrode 129 is formed. In addition, in the periphery of the dielectriclaminate reflecting layers 128, a window-shaped electrode 130 is formed.Thus, all the parts formed on the Ga—As substrate compose an opticalresonator. A light beam which is generated on the active layer 125reciprocatively travels between the upper reflecting layer 127 and thelower reflecting layer 123 in the direction perpendicular to the surfaceof the device substrate 122. As a result, the optical axis of the laserbeam 31 is nearly perpendicular to the surface of the device substrate122. In the periphery of the optical resonator, a group II-VI compoundsemiconductor is embedded as an embedded layer 132. As the group II-VIcompound semiconductor, it is preferable to use a group II-VI compoundwhich contains two, three, or four elements selected from both group IIelements Zn, Cd, and Hg and group VI elements O, S, Se, and Te. Inaddition, it is preferable to match the lattice constant of the compoundwith that of the semiconductor layers composed of the clad layer 124,the active layer 125, and the clad layer 126. Since the electricresistance of the group II-VI compound semiconductor is very large, thecurrent is effectively closed in the optical resonator. In addition,since the refractive index of the embedded layer 132 differs from thatof the Al—Ga—As semiconductor layer, the beam which travels in theoptical resonator in the direction just or nearly perpendicular to thesurface of the device substrate 122 is totally reflected at the with theembedded layer 132. Therefore, when such a semiconductor laser is used,the laser oscillation starts with a very small amount of current incomparison with the conventional laser. In other words, the thresholdvalue of the semiconductor laser of the present invention is lower thanthat of the conventional one. In addition, the amount of lost heat issmall.

In addition, since the sectional area (near field pattern) of the laserbeam emitting portion of the surface light emitting type semiconductorlaser is larger than that of the conventional edge emitting typesemiconductor laser, the spread angle of the laser beam is small.Although the value of the spread angle just depends on the area of thelight emitting window, the area can be precisely controlled by anetching process or the like. Thus, the spread angle can be keptconstant. Moreover, the ratio of the length and width of the spreadangle of a laser beam, namely, the ratio of the longer diameter andshorter diameter of a beam whose cross section is elliptic can be freelyset with the shape of the light emitting window. For example, when theshape of the window is a perfect circle, a laser beam with a circularcross section or an isotropic spread angle can be obtained. Thus, theastigmatic difference on the cross section in the direction of theoptical axis of a beam is small.

However, in a conventional laser beam printer, the image spot of a laserbeam on the image bearing member is sometimes in an elliptic shape wherethe scanning direction of the laser beam accords with the minor axisthereof. Since the spot moves in the scanning direction for the lightingtime of the laser beam and thereby the image is expanded, the imageshould be compensated. Thus, the shape of the cross section of the laserbeam which enters the image forming optical system is preferablyelliptic. In the surface light emitting type semiconductor laser, sincethe elliptic ratio of the laser beam can be freely controlled, the laserbeam with a cross section whose major axis accords with the scanningplane and whose ratio of the major axis and the minor axis isappropriate can be entered into the image forming optical system.

As the sectional area of the optical resonator increases on the surfaceof the device substrate 122 of the surface light emitting typesemiconductor laser 121 becomes large, laser oscillations of high ordermodes along with zero-th mode start. The distribution of the amount oflight of the image spot has several peaks. Thus, this situation is notsuitable for forming a static latent image on the image bearing member105. To prevent this situation, a plurality of small optical resonatorsare closely disposed and the phase of each laser beam oscillated issynchronized. As a result, the light emitting portions 121 a which arelarge in size and oscillated in the zero-th mode can be obtained.

FIG. 15 shows a partial sectional view of a light emitting portion 121 aof the phase-lock surface-light-emitting type semiconductor laser 121.In this semiconductor laser 121, a plurality of optical resonators aredisposed at very short intervals. The bottom of the embedded layer 132does not reach the active layer 125. Thus, rays which leak out from theadjacent optical resonators adversely affect each other through the cladlayer 126 below the embedded layer 132. Thus, laser beams are oscillatedwith the same phase. As a result, the plurality of adjacent opticalresonators operate as if a single optical resonator works. Since thewave faces of beams emitted from the optical resonators are matched,these optical resonators work as a surface laser emitting source. Thus,the apparent area of the light emitting portion 121 a becomes large. Thespread angle of each laser beam is as small as 2 degrees or below infull width at half maximum.

In the phase-lock surface-light-emitting type semiconductor laser, thespread angles of laser beams are small in comparison with theconventional semiconductor laser. With respect to this characteristic,the relation between the embodiment of the present invention and theembodiment of the related art will be described. When the spread angleof a laser beam is 2 degrees in full width at half maximum and the laserbeam is entered into an image forming optical system with a beamdiameter of 2 mm, the focal length fc of a collimator lens 102 becomesapproximately 35 mm. Since the focal length fc of the collimator lens102 can become long, the adjustment allowance of the distance of thecollimator lens 2 to the semiconductor laser 121 is increased.

Further, by excessively decreasing the spread angle of each laser beam,after the laser beam travels from the semiconductor laser to therotating polygon mirror 103 to the image forming lens 104, the laserbeams is not largely spread. Thus, even on the incident plane of theimage forming lens 104, the size of the laser beam can be decreasedenough to obtain a predetermined diameter of an image spot. In otherwords, the collimator lens which collimates a laser beam with apredetermined collimate diameter is not required as opposed to theconventional laser scanning optical system. However, depending on thedefection angle of the rotating polygon mirror 103, the length of theoptical path varies, so does the size of the laser beam entered into theimage forming lens 104. Thus, there should be provided an optical systemwhich compensates the size of the laser beam. Nevertheless, the imageforming lens 104 can be easily provided with such an optical function.Thus, the number of constructional parts of the overall optical systemcan be decreased.

In the surface light emitting type semiconductor laser, generally anemitted laser beam is linearly polarized. The direction of thepolarization depends on the shape of the plane of the optical resonatoron the device substrate. The plane of polarization nearly accords withthe longitudinal direction of the shape of the plane of the opticalresonator. For example, when the optical resonator is in an ellipticshape, the direction of the major axis thereof becomes the plane ofpolarization. As described above, in the phase-lock type semiconductorlaser, the light emitting portion is composed of a plurality ofphase-lock optical resonators. The number of optical resonators is forexample four. At this point, the shape of the cross section of the laserbeams emitted is a composite shape thereof. Thus, in accordance with thearrangement of the optical resonators, the shape of the cross section ofthe composite laser beam can be freely set. In this case, theorientation of the plane of polarization depends on the shape of theplane of each optical resonator. Thus, in the case where a compositelaser beam in a elliptic shape is -obtained, the major axis and thedirection of the plane of polarization can be independently set.

FIG. 16(a) schematically shows this situation. This figure is a planview of a light emitting portion 142 of a semiconductor laser, the lightemitting portion 142 being seen from the beam emitting side. The lightemitting portion 142 is composed of four optical resonators 41 whichphase-synchronously oscillate. The plane of polarization 143 of a laserbeam emitted from each optical resonator 41 is inclined by 45 degrees asshown in FIG. 16(a). The major axis of the composite elliptic laser beamaccords with the vertical direction of the light emitting portion. Inaddition, as shown in FIG. 16(b), when the planes of polarization 43 ofthe laser beams emitted from the optical resonators 41 are oriented atdifferent angles, the composite laser beam nearly becomes circularlypolarized light.

As described above, in the conventional laser beam printer, an imagespot on the image bearing member is often in an elliptic shape where theminor axis thereof accords with the scanning direction. When thesemiconductor laser is disposed in such a way that the orientation ofthe plane of polarization of a laser beam from each optical resonator isinclined to the direction of the major axis of the elliptic crosssection of the composite laser beam by 45 degrees and the major axis ofthe composite beam is matched with the scanning direction, the plane ofpolarization of the laser beam from each optical resonator is inclinedto the beam scanning plane by 45 degrees. As a result, the plane ofpolarization is inclined to the rotating axis of the rotating polygonmirror 103 by 45 degrees. Thus, as shown in FIG. 19, the difference ofreflectances of laser beams according to incident angles thereof to therotating polygon mirror becomes small. This situation also applies to alaser beam with a cross section of an elliptic shape, the beam beingcircularly polarized light. In some optical systems, the minor axis of alaser beam emitted from the semiconductor laser is matched with thescanning direction. In this situation, the same effects can be obtained.

It should be appreciated that the above-mentioned embodiment is anexample of the present invention. For example, by using a Galvano mirroror a hologram disk instead of the rotating polygon mirror, the sameeffects can be obtained. In addition, regardless of whether thecollimator lens, the tilt angle compensation lens, and/or the imageforming lens is used or not, the same effects can be obtained. Moreover,even if the construction and/or relative positions of these lenses arechanged, the same effects of the present invention can be likewiseachieved.

Furthermore, the image forming apparatus of the present invention can beused for facsimile machines, display units, and so forth as well asprinting units such as printers and copy machine.

2-3 Effects

As described above, in the image forming apparatus of the presentinvention, since a surface light emitting type semiconductor laser isused, the following results can be obtained.

(1) Since the spread angle of each leaser beam is small and the distancebetween the collimator lens and the semiconductor laser is large, theadjustment allowance in the direction of the optical axis of thecollimator lens is increased and thereby the producibility of theapparatus is improved. Moreover, without influences of ageddeterioration and temperature fluctuation in operation, an image can beexposed with a predetermined spot diameter. As a result, the imagequality is improved.

(2) Since the astigmatic difference of the surface light emitting typesemiconductor laser is low as a characteristic thereof, the ellipticshape of the cross section of a beam (ratio of major axis and minoraxis) can be freely set. Thus, without necessity of an optical systemwhich compensates the astigmatic difference, the beam can be preciselyshaped.

(3) Since a group II-VI compound semiconductor is used for the surfacelight emitting type semiconductor laser as an embedded layer, the laseroscillation can be accomplished with a low threshold current value.Thus, the bad influence of heat generated by the device against thelaser characteristics can be reduced.

(4) When a phase-lock surface-light-emitting type semiconductor laserwith a plurality of optical resonators which emit phase-locked laserbeams is used as the surface light emitting type semiconductor laser,the spread angle of each laser beam can be further decreased. In somecases, the collimator lens can be omitted. Thus, the construction of theoptical system can be further simplified.

(5) Since the light emitting portion can be composed of a plurality ofoptical resonators whose plane of polarization can be freely set, when acomposite laser beam with an elliptic cross section is used, thedirection of the plane of polarization of the laser beam can be setfreely and independent from the direction of the major axis of the crosssection of the composite laser beam. As a result, the fluctuation of theamount of light according to the position in the scanning direction ofthe laser beam due to the difference of incident angle of the laser beamto the polygon mirror can be minimized.

Section 3 Third Embodiment of Image Forming Apparatus

3-1 Comparison with Related Art

Before describing a third embodiment of the present invention, so as toeasily understand the conception thereof, the related art thereof willbe described.

FIG. 27 shows a sectional view of an optical path of a conventionalimage forming apparatus. The figure illustrates a plane which isperpendicular to a scanning plane of a image bearing member of an imageforming apparatus and which contains an optical axis. That is, thefigure shows a sectional view of the optical path on a sub scanningplane. In the figure, the optical axis is bolded back at a reflectingsurface 208 of a rotating polygon mirror. In the figure, a laser beamwhich is emitted from a semiconductor laser 201 is radiated with aspread angle θ. This beam is shaped to a nearly parallel beam by acollimator lens 202 of a focal length fc. Each beam is collected on thereflecting surface 208 of the rotating polygon mirror by a tilt anglecompensation lens 207. The beam which is deflected by the rotatingpolygon mirror 208 becomes a parallel beam after passing through asecond tilt angle compensation lens 207′. Thereafter, the beam is imagedas a spot 206 on a image bearing member 211 by a scanning lens 204 of afocal length fi. On a plane in parallel with the scanning plane, sincethe tilt angle compensation lenses 207 and 207′ do not have opticalpowers, the beam is kept parallel. In other words, the beam is imaged asa line images on the reflecting surface 208 of the rotating polygonmirror 208.

Next, the operations of the tilt angle compensation lenses 207 and 207′will be described. The inclination of each reflecting surface 208 of therotating polygon mirror has an error of the order of several ten secondsin angles to the rotating axis. Thus, the image position of the beamreflected to this surface has a deviation in the sub scanning directionon the surface of the image bearing member due to the effect of “opticallever.” This deviation is too large to ignore in comparison with pitchesof scanning lines. To prevent this deviation, as disclosed in JapanesePatent Laid-Open Publication Serial No. SHO 48-49315, a tilt anglecompensation lens 207′ which allows each reflecting surface of thepolygon mirror and the surface of the image bearing member 211 (thesurface on which an image is formed) to be placed in optical conjugatepositions. This tilt angle compensation lens 207′ is normally acylindrical lens or a toric lens which has an optical power only on asub scanning plane. Even if the reflecting surface is inclined, the beamis always imaged in the same position on the image forming surface.

FIG. 28 is a schematic diagram showing the conception of a so-callededge emitting type semiconductor laser which has been widely used. Asshown in the figure, the spread angle of a beam on the plane which hasthe optical axis and is in parallel with the contact surface largelydiffers from that on the plane which has the optical axis and isperpendicular to the contact surface. The spread angle θp on the planein parallel with the contact surface of a conventional laser diode isapproximately 10 degrees in full width at half maximum. On the otherhand, the spread angle θt on the plane perpendicular to the contactsurface is as large as approximately 30 degrees in full width at halfmaximum due to an influence of diffraction.

However, when there is a difference between the spread angle of a beamon the contact surface and that on the plane perpendicular thereto asdescribed above, if the beam is collimated by the collimator lens 202,the cross section of the resultant laser beam becomes a largelycompressed elliptic shape. When the parallel beam where the ratio of thelonger diameter and shorter diameter is large is imaged on the imagebearing member 211 by the scanning lens 204, the relation of ratio ofthe longer diameter and shorter diameter of the parallel beam becomesreverse.

On the other hand, as disclosed in Japanese Patent Laid-Open PublicationSerial No. SHO 52-119331, on the image plane, that is, on the imagebearing member, the image spot is preferably an ellipse with acomparatively shorter minor axis in the scanning direction. Since thelaser is lit at predetermined intervals of pulses, the spot diameter inthe scanning direction should be smaller than that in the sub scanningdirection so as to compensate the traveling distance of the laser beam.

Thus, to image a beam of a large ratio of spread angles as a spot with apredetermined ratio of longer diameter and shorter diameter, ananamorphic optical system where the optical characteristics in thescanning direction differ from those in the direction perpendicularthereto should be disposed on the optical path somewhere from thesemiconductor laser 201 to the image bearing member 211.

The technique for providing this characteristic to the above-mentionedtilt angle compensation optical system has been widely used. In FIG. 27,since the tilt angle compensation lenses 207 and 207′ are afocal in thesub scanning direction, when the distance between the reflecting surfaceand one lens differs from that between the reflecting surface andanother lens, a beam expander which works only in the sub scanningdirection can be formed.

However, such a tilt angle compensation optical system is always alongitudinal cylindrical lens or a toric lens, it leads to a difficultyof production or rise of production cost. In addition, the optical axisof the tilt angle compensation lens 207 in front of the rotating polygonmirror should be accurately adjusted. These drawbacks prevent theproducibility of the image forming apparatus and the reliability thereoffrom being improved.

As described above, a major factor which causes an angular error of alaser beam on the sub scanning plane is the machine accuracy of theangle of each reflecting surface of the rotating polygon mirror. Inother words, the dynamic vibration of the rotating axis of the rotatingpolygon mirror is not a critical matter. Thus, by rotating a singlemirror surface rather than the rotating polygon mirror, the major factorof this matter can be solved. Thus, in the conventional image formingapparatus, the tilt angle compensation lenses are not necessary.

In addition, since such a rotating mirror has only one reflectingsurface, the machining work thereof is easy and the moment of inertia issmall. Thus, this rotating mirror can have a resistance to the vibrationthereof.

The concept of this rotating single-surface mirror is not novel.However, one rotation of this mirror allows only one line to be scanned.In other words, this mirror is not suitable for use in the so-calledlaser printer.

Now, consider the following situation. The pitch of scanning lines is300 dpi (300 dots per inch (=25.4 mm), that is, the number of scanninglines is 300). The paper size is A4. Papers of this size are loaded inthe longitudinal direction of the laser printer. 10 papers are printedper minute. In this situation, when a rotating polygon mirror with sixsurfaces is used, the number of rotations is approximately 7000 rpm(rotations per minute). It is said that the upper limit of the number ofrotations of a rotating polygon mirror using a ball bearing is in therange from 12000 to 14000 rpm. When one reflection mirror which isrotated at 12000 rpm is used, the number of papers which can be printedis at most three.

On the other hand, when the tilt angle compensation lenses 207 and 207′are removed, as described above, the function as the anamorphic beamexpander which images a laser beam with a large ratio of spread anglesas a spot with a particular elliptic ratio on the image bearing memberis lost.

3-2 Construction of the Present Invention

FIG. 21 is a schematic diagram showing an image forming apparatusaccording to the present invention. In the process for obtaining a printresult on an image transfer material 251, namely, in theelectrophotographic process, an organic photoconductor (OPC) with anincreased sensitivity in a longer wavelength region has been widelyused. This image bearing member 205 is charged to a predeterminedsurface potential by a charger 252. Thereafter, a laser beam scanningunit 253 performs a light writing process, that is, a light exposingprocess. In accordance with image information from the laser beamscanning unit 253, a plurality of laser beams 254 whose lightintensities are individually modulated are scanned on image bearingmember 205 in the axial direction thereof, thereby generating electriccharges which neutralize the surface potential only for the exposedportion. Thus, the absolute value of the surface potential of thisportion becomes low. As a result, on the image bearing member 205, adistribution of surface potentials in accordance with the image, thatis, a static latent image is formed. A developing unit 255 selectivelyadheres a developing agent in accordance with the surface potentials tothe image bearing member 205. Thus, the static latent image isdeveloped. This developing agent is transferred to a transfer material251 (normally, a paper) by a transferring unit 256. The developing agenton the transfer material 251 is fixed with a thermal pressure by afixing unit 257. Thereafter, the transfer material 251 is unloaded fromthe apparatus.

FIG. 20 is a schematic diagram showing the construction of a laser beamscanning unit of the present invention. In the laser beam scanning unit253 shown in FIG. 21, the laser beams 254 are folded back and downwardlyemitted. However, in this example, the illustration of the laser beamsis simplified. The technique for scanning a image bearing member with aplurality of laser beams is referred to as a multi-beam laser scanningtechnique. A plurality of laser beams emitted from a plurality of lightemitting portions 221 a of a semiconductor laser array 221 arecollimated to laser beams with predetermined diameters by a collimatorlens 2. The laser beams are entered into a rotating mirror 218 with onereflecting surface. As the rotating mirror 218 rotates, these laserbeams are deflected. The laser beams which pass through a scanning lens204 are imaged as spots 6 on the image bearing member 205. The lightingand the amount of light of each of the light emitting portions 221 a arediscretely controlled by a control unit 260.

The scanning lens 204 has two major functions. The first function of thescanning lens 204 is a so-called “fθ” function which converts thescanning motion of iso-angular velocity of a rotating mirror 218 intothe scanning motion of iso-linear velocity on the image bearing member205. The second function of the scanning lens 204 is the compensatingfunction for curvature of field. With this function, the image pointmoves forward or backward in the direction of optical axis depending onthe scanning angle so that the image plane becomes flat.

Each of the collimator lens 202 and the scanning lens 204 has aniso-tropic optical characteristic on the entire plane including theoptical axis thereof. In other words, on the entire plane including theoptical axis, the focal length and curvature of the collimator lens 202are the same as those of the scanning lens 204. Thus, these lenses 202and 204 are non-anamorphic lenses each other.

In this embodiment, since the number of laser beams is four, if thenumber of rotations of the rotating mirror 218 is not changed, fourtimes the scanning speed can be obtained. As described above, when therotating mirror which can be rotated at the maximum speed currentlyavailable is used, the print speed of 10 papers (in A4 size) per minutecan be obtained. This speed is satisfactory for personal-use laserprinters. When the number of laser beams is further increased, the speedsuitable for business-use laser printers can be obtained.

Each light emitting portion 221 a of the semiconductor laser array 221is independently controlled in accordance with image data to be writtento each scanning line. Each light emitting portion 221 a emits amodulated laser beam. Thus, parallel data is transferred from an imagedata storage portion (not shown in the figure) to the semiconductorlaser array 221.

As will be described later, the position in the scanning direction ofeach image spot differs one by one. Thus, the modulation timing of eachlaser beam is delayed in accordance with the amount of deviation of thisposition.

As shown in FIG. 22, now, for simplicity, consider a multi-beam laserscanning optical system using two laser beams, a convex collimator lens,and a convex image forming lens. The two laser beams spaced apart by dand emitted from the semiconductor laser array 221 are collimated by thecollimator lens 202 of a focal length fc. Since the semiconductor laserarray 221 is disposed at an object side focal point of the collimatorlens 202, the two laser beams are intersected at an image side focalpoint F thereof. When a spot 206 of d₀=100 μm is formed on the imageplane 211, if fi is 200 mm, the diameter Wc of the beam entered into thescanning lens, that is, the collimate diameter, is given by the formula(5). The spot diameter or the beam diameter is a diameter where theintensity of the cross section of a beam is the power of the peakintensity times (1/e²). The distribution of this intensity accords witha Gaussian distribution. $\begin{matrix}{{WC} = {d_{0}\sqrt{1 + ( \frac{4\quad \lambda \quad {fi}}{\pi \quad d_{0}^{2}} )^{2}}}} & (5)\end{matrix}$

However, λ is the wavelength of the laser, which is 780 nm. On the otherhand, the focal length fc of the collimator lens 202 depends on thespread angle of the laser beam emitted from the semiconductor laserarray 221. The focal length fc is given by the following formula.$\begin{matrix}{{fc} = \frac{WC}{\theta}} & (6)\end{matrix}$

where θ is defined by 1/e2 in full width along with the diameter of thebeam.

In addition, due to the arrangement of each element of the scanningunit, a particular distance h should be disposed between the collimatorlens 202 and the reflecting surface of the deflecting unit. Moreover,when n laser beams are disposed in line, the distance q of each adjacentreflection position of n laser beams on the reflecting surface 208 isgiven by the following formula. $\begin{matrix}{q = {{d \cdot \frac{h - {fc}}{fc}}( {n - 1} )}} & (7)\end{matrix}$

Next, an edge emitting type semiconductor laser where a laser beam isemitted from the edge of a device substrate has been widely used for asemiconductor laser array. As described in the paragraph of the relatedart shown in FIG. 28, a laser beam emitted is diffracted in thedirection perpendicular to the substrate of the semiconductor laserarray 201. Thus, the laser beam is spread with an angle of approximately30 degrees in full width at half maximum. At this point, the focallength fc of the collimator lens 202 becomes approximately 3 mm. Whenthe number n of laser beams is four and the distance h from thecollimator lens 202 to the reflecting surface 208 is 100 mm, thedistance q of each adjacent beam reflection position becomes 9.7 mm.Thus, the size of the reflecting surface should be increased by thedistance q of each adjacent beam reflection position. In this situation,according to the present invention, since there is only one reflectingsurface, the size of the reflecting surface can be more easily increasedthan the case where the rotating polygon mirror is used.

However, in the semiconductor laser array 221, it is preferable to use aso-called surface light emitting type semiconductor laser. In thesurface light emitting type laser array 221, since the sectional area ofthe light emitting portions 221 a is larger than that of theconventional edge emitting type semiconductor laser array, the spreadangles of the laser beams are small. Although the value of the spreadangle just depends on the area of the light emitting window, the areacan be precisely controlled by an etching process or the like. Thus, thespread angle can be kept constant. For example, a laser beam with aspread angle of approximately 8 degrees in full width at half maximumcan be satisfactorily obtained. In addition, with the surface lightemitting type semiconductor laser, since current and light can beeffectively closed in an optical resonator of the laser, the amount ofheat generated by each light emitting portion can be decreased.Moreover, when a plurality of light emitting portions are adjacentlydisposed, mutual optical, electrical, and thermal interferences can bedecreased. Thus, the distance of each adjacent light emitting portioncan be decreased in comparison with the conventional semiconductorlaser.

When the above-mentioned formula (6) is applied to the surface lightemitting type semiconductor laser with a beam spread angle of 8 degrees,the focal length fc of the collimator lens becomes approximately 8 mm.In addition, since the distance d of each adjacent light emittingportion on the semiconductor laser array can be set to as narrow as 50μm, when the number n of beams is 4 and the distance h from thecollimator lens 202 to the reflecting surface 208 is 100 mm, by applyingthe above-mentioned formula (7), the distance q of each adjacentreflection position of the four beams on the reflecting surface becomesapproximately 1.73 mm. Thus, the distance q of each adjacent beamreflection position is not noticeable against the collimate diameter Wcof the beams.

In particular, consider the case where the spot diameter on the imagebearing member is set to 50 μm so as to form an image with a much higherresolution. By applying the above-mentioned formula, the collimatediameter Wc is doubled (namely, approximately 4 mm). Thus, the focallength fc of the collimator lens is also doubled. The distance q of eachadjacent beam reflection potion on the reflecting surface is halved.

When each beam is tracked, the distance of each adjacent beam in anyposition on the optical axis is much smaller than the collimatediameter. Thus, even if an optical system deals with a plurality oflaser beams, it is possible to design the optical system by consideringonly one typical beam. Thus, the design work of the laser scanningoptical system is very simplified.

In addition, since the focal length of the collimator lens of thisembodiment is larger than that of the case where the conventionalsurface light emitting type semiconductor laser is used, the allowanceof the distance between the semiconductor laser and the collimator lensin the optical direction can be increased.

In a surface light emitting type semiconductor laser array suitable forthe multi-beam scanning technique, a surface light emitting typesemiconductor laser array where a group II-VI compound semiconductor isembedded in the periphery of the light emitting portion is morepreferable.

FIG. 23 is a sectional view showing one of the light emitting portions221 a of the surface light emitting type semiconductor laser array 221.As shown in the figure, on the Ga—As device substrate 222, asemiconductor laminate reflecting layer 223 is formed. The reflectinglayer 223 is composed of several ten layers of two types of Al—Ga—Ascompounds. On the reflecting layer 223, a clad layer 224, an activelayer 225, a clad layer 226, and a contact layer 227 are disposed, eachof which is composed of Al—Ga—As compounds. On the contact layer 227, aSiO₂ dielectric laminate reflecting layer 228 is formed. On the entirerear surface of the Ga—As substrate, a window-shaped electrode 229 isformed. In addition, in the periphery of the dielectric laminatereflecting layer 228, a window-shaped electrode 230 is formed. Thus, allthe parts formed on the Ga—As substrate compose an optical resonator.

A light beam which is generated on the active layer 225 reciprocativelytravels between the upper reflecting layer 227 and the lower reflectinglayer 223 in the direction perpendicular to the surface of the devicesubstrate 222. Thus, the light beam is oscillated. As a result, theoptical axis of the laser beam 231 is nearly perpendicular to thesurface of the device substrate 222.

In the periphery of the optical resonator, a group II-VI compoundsemiconductor is embedded as an embedded layer 232. As the group II-VIcompound semiconductor, it is preferable to use a group II-VI compoundwhich contains two, three, or four elements selected from both group IIelements Zn, Cd, and Hg and group VI elements O, S, Se, and Te. Inaddition, it is preferable to match the lattice constant of the compoundwith that of the semiconductor layers composed of the clad layer 224,the active layer 225, and the clad layer 226. Since the electricresistance of the group II-VI compound semiconductor is very large, thecurrent is effectively closed in the optical resonator. In addition,since the refractive index of the embedded layer 232 differs from thatof the Al—Ga—As semiconductor layer, the beam which travels in theoptical resonator in the direction just or nearly perpendicular to thesurface of the device substrate 222 is totally reflected at theinterface with the embedded layer 232. Thus, the beam is effectivelyclosed in the optical resonator. Therefore, when such a semiconductorlaser array 221 is used, the laser oscillation starts with a very smallamount of current in comparison with the conventional laser array. Inother words, the threshold value of the semiconductor laser array 221 islower than that of the conventional one. Thus, the amount of lost heaton the device substrate is small. In FIG. 23, a diode is formed on theGa—As substrate 222. The light generated in the active layer 225reciprocally travels between the reflection layers 223 and 228. Thus, alaser oscillation takes place. A laser beam 231 is emitted from thereflection layer 228 whose reflectance is smaller than that of thereflection layer 228 in the direction perpendicular to the surface ofthe device substrate.

As described in the paragraph of the related art, in the conventionallaser beam printer, since an image spot of a laser beam formed on aimage bearing member is in an elliptic shape where the minor axisthereof accords with the scanning direction, the cross section of thelaser beam entered into the scanning lens is preferably an ellipticshape where the major axis thereof accords with the scanning direction.

However, according to the present invention, the tilt angle compensationoptical system is not used. As described above, when the edge emittingtype semiconductor laser array is used, by using a collimator lens withan anamorphic characteristic, it is necessary to correct the ratio ofthe major diameter and minor diameter of the elliptic cross section of alaser beam which is entered into the scanning lens. Such a collimatorlens is not difficult to produce. Thus, this lens does not decrease theadvantages of the present invention.

However, in the surface light emitting type semiconductor array, theelliptic ratio of the cross section of a laser beam emitted can befreely controlled without necessity of a special optical system. Thus, alaser beam in an elliptic shape whose major axis accords with thescanning plane and which has a proper elliptic ratio of major axis andminor axis can be entered into the scanning lens. In other words, toobtain an ideal elliptic ratio of an image spot, the surface lightemitting type semiconductor laser array is most suitable.

In addition, in the surface light emitting type semiconductor laser,since the light emitting portions can be disposed at any positions wherethey do not interfere each other, they can be two-dimensionally disposedon the device. Now, in an exposing system which scans the image bearingmember with four laser beams, consider the relation between scanninglines and image spots. In addition, assume the case where four adjacentscanning lines are drawn with one scanning sequence. When the imagespots 206 are disposed as shown in FIG. 24(a), the angle and thedistance of each adjacent laser beam can be decreased in comparison withthe case where they are disposed in line as shown in FIG. 24(b). Thus,the size of the reflecting surface of the reflector and the sizes ofother optical systems can be according decreased.

In the above example, the case where four laser beams are used wasconsidered. When the number of laser beams is further increased, thelight emitting portions can be freely disposed on the semiconductorlaser array so that the spots are disposed at the closest positions onthe image bearing member. As a result, more significant effects can beobtained than the above-mentioned construction. In FIG. 24(c), anexample of the arrangement of image spots according to scanning lines inthe case where eight laser beams are used is shown. In other words, evenif there is provided one reflecting surface of the rotating mirror, byincreasing the number of beams, a satisfactorily high print speed can beobtained.

According to this embodiment, in the scanning optical system, theoptical characteristics in the main scanning direction are the same asthose in the sub scanning direction. Thus, the positions of image spotson the image bearing member are similar to those on light emittingportions of the semiconductor laser array.

Next, with reference to a plan view of FIG. 25, a deflecting unit usedin the present invention will be described. The deflecting unit isprovided with one reflecting surface 215. The reflecting surface 215 ismounted on a rotating portion of a motor 216. The reflecting surface 215is rotated at a constant speed. As described above, so far, theconventional polygon mirror has had restrictions in the machining methodand construction so as to maintain the mutual angular accuracy of eachreflecting surface. Generally, the rotating polygon mirror is composedof a single metal rod which is ground in a polygon shape. Each surfaceis mirror-finished and then coated. However, in the case of thedeflecting unit having a single reflecting surface, without necessity ofthe above-mentioned grinding process, by adhering a flat glass platewith an evaporated mirror film to the rotating portion, the deflectingunit can be produced. Thus, the production cost is very low.

The rotating portion including the reflecting surface is lighter thanthe conventional polygon mirror. Thus, the rotating portion has a largeresistance to vibrations caused by the rotations thereof. This rotatingportion is designed to have a dynamic balance to the rotating axis.Alternatively, if necessary, the dynamic balancing means is added to therotating portion.

In addition, the rotating portion is designed to allow the center of thereflecting surface 215 to accord with the rotating shaft A. When a laserbeam 217 is entered in the vicinity of the rotating axis A on thereflecting surface 15, even if the reflecting surface 215 is rotated,the laser beam 217 is positioned almost at one point on the reflectingsurface 215. Thus, the size of the reflecting surface becomes very smallin comparison with the conventional rotating polygon mirror.

Moreover, in the case where there is provided only one reflectingsurface, angles at which beams can be deflected are much wider than thecase where the polygon mirror is used. For example, in the case where amirror with six reflecting surfaces as shown in FIG. 26 is used,provided that the collimate diameter Wc of the laser beam 217 is 0, thelimit of the scanning angle α is 120 degrees regardless of the size ofthe reflecting surface. On the other hand, the effective scanning angleof a scanning optical system which can be conventionally designed is atmost approximately 90 degrees. Moreover, in consideration of thecollimate diameter Wc of a laser beam entered into the reflectingsurface and the space of a detecting unit which detects the scanningstart position of the beam, the above-mentioned mirror with sixreflecting surfaces does not provide a sufficient scanning angle of thedeflecting unit. Thus, the effective scanning angle of the scanning lensis narrowed.

On the other hand, in the case of single reflecting surface, if therotating axis is placed on the reflecting surface or the size of thereflecting surface is infinite, the effective scanning angle logicallybecomes 360 degrees. Thus, in this case, a scanning lens with a largescanning angle can be used and the size of the scanning optical systemcan be decreased.

Further, in the case of single reflecting surface, the period for whichthe reflecting surface is actually used for one scanning sequence isapproximately 10% of the period of one rotation thereof. The remainingperiod is not involved in the scanning sequence. If a laser beam isradiated in the remaining period, it is emitted to the rear of thereflecting surface. Thus, unexpected reflection light is imaged on theimage bearing member. Thus, a circuit which stops the laser oscillationin the unnecessary period is provided.

Alternatively, by using the period which is not involved in the scanningsequence, after the laser oscillation is started, the amount of light isdetected and thereby the laser drive current is set so that apredetermined amount of light is obtained. To prevent a laser beam of anunnecessary reflection from reaching the image bearing member, it ispreferable to properly design the angle of peripheral members of thedeflecting unit or perform a surface treatment which preventsunnecessary light from reflecting.

The above-mentioned embodiment is only one embodiment of the presentinvention. Even if the constructions of the collimator lens and thescanning lens and the relation of the relative positions thereof arechanged, the same effects of the present invention can be obtained. Inaddition, it should be noted that even if the rotating mirror isconstructed and/or produced by using a different method such as aplastic injection molding method, the similar effects of the presentinvention can be attained. Moreover, besides the rotating mirror whichrotates in one direction at a constant speed, with a so-called galvanomirror which rotatably vibrates, the effects of the present inventioncan be similarly acquired.

Further, the construction of the device of the surface light emittingtype laser described in the above-mentioned embodiment is only anexample which can be accomplished. In the condition where thecharacteristics of the spread angle of each emitted beam, the distanceof each light emitting portion, and so forth are the same as those ofthe above-mentioned embodiment, the same effects of the presentinvention can be obtained regardless of the construction thereof.

It is obvious that the scope of the applications of the image formingapparatus of the present invention includes facsimile machines anddisplay units as well as printing apparatus such as the printers andcopy machines. In these apparatuses and units, the same effects of thepresent invention can be attained.

3-3 Effects

As described above, in the image forming apparatus of the presentinvention, since the rotating mirror with one flat reflecting surface isused as a deflecting unit, the rotating portion of the deflecting unitbecomes small in size and light in weight. Thus, the deflecting unit canbe easily produced. In addition, the dynamic vibration characteristicsof the deflecting unit is improved. Furthermore, since the tilt anglecompensation optical system and the anamorphic optical system areomitted, a scanning optical system in a very simple construction whichis easy to assemble and adjust can be accomplished. Moreover, since themulti-beam system is used, the scanning speed which is the same as thatof the conventional apparatus can be maintained.

According to the present invention, in particular, when the surfacelight emitting type semiconductor laser array is used, with thecollimator lens and the scanning lens other than a new additionaloptical system, the size of the reflecting surface can be decreased. Inaddition, the elliptic ratio of the image spot on the image plane can befreely set.

Section 4 Fourth Embodiment of Image Forming Apparatus

4-1 Comparison with Related Art

Before describing a fourth embodiment of the present invention, so as toeasily understand the conception thereof, the related art thereof willbe described.

FIG. 33 shows a sectional view of an optical path of a conventionalimage forming apparatus. In the figure, a laser beam which is emittedfrom a semiconductor laser 301 is radiated with a spread angle θ. Thisbeam is shaped to a nearly parallel beam by a collimator lens 302 of afocal length fc. Each beam is collected on the reflecting surface 308 ofthe rotating polygon mirror by a tilt angle compensation lens 307. Thebeam which is deflected by the rotating polygon mirror 308 becomes aparallel beam after passing through a second tilt angle compensationlens 307′. Thereafter, the beam is imaged as a spot on a image bearingmember by a scanning lens 304 of a focal length fi. On a plane inparallel with the scanning plane, since the tilt angle compensationlenses 307 and 307′ do not have optical powers, the beam is keptparallel. In other words, the beam is imaged as a line image on thereflecting surface 308 of the rotating polygon mirror.

Next, the operations of the tilt angle compensation lenses 307 and 307′will be described. The inclination of each reflecting surface 308 of therotating polygon mirror has an error of the order of several ten secondsin angles to the rotating axis. Thus, the image position of the beamreflected to this surface has a deviation in the sub scanning directionon the surface of the image bearing member due to the effect of “opticallever”. This deviation is too large to ignore in comparison with pitchesof scanning lines. To prevent this deviation, as disclosed in JapanesePatent Laid-Open Publication Serial No. SHO 48-49315, a tilt anglecompensation lens 307′ which allows each reflecting surface of thepolygon mirror and the surface of the image bearing member (the surfaceon which an image is formed) to be placed in optical conjugatepositions. This tilt angle compensation lens 207′ is normally acylindrical lens or a toric lens which has an optical power only on asub scanning plane. Even if the reflecting surface is inclined as shownwith 308′ of FIG. 33, the beam is always imaged in the same position onthe image forming surface.

However, in recent years, as the skills for using computers areimproved, the needs of high output speeds of image forming apparatusesare becoming strong. Accordingly, the modification of the apparatuses isactively performed. However, a deflecting unit using a rotating polygonmirror deflects one laser beam per facet and draws one scanning line.Thus, when the number of scanning lines in a predetermined time periodis increased, provided that the number of surfaces of the rotatingpolygon mirror is not changed, the number of rotations thereofincreases. On the other hand, provided that the number of rotations isnot changed, the number of surfaces of the rotating polygon mirrorincreases. When the number of rotations of the rotating polygon mirroris increased, one of bearings using dynamic or static pressure of gas orliquid should be used. However, these bearings are expensive in cost anddifficult to handle. Thus, so far, these bearings could not be used forlaser beam printers. On the other hand, when the number of surfaces ofthe rotating polygon mirror is increased, the deflection angle becomessmall. Thus, the length of the optical path following the deflectingunit becomes long and the collimate diameter of the laser beam enteredinto the image forming optical system proportionally becomes large.Thus, the sizes of the lens and the rotating polygon mirror becomelarge. In particular, when a high resolution of the image formingapparatus is also required along with a high output speed thereof, sincethe number of scanning lines also increases, a larger number ofrotations and a longer optical path are further required. This situationalso applies to an deflecting unit which is other than the rotatingpolygon mirror. In this situation, the scanning frequency becomes highand the length of the optical path following the deflecting unit becomeslong. To solve this problem, a new exposing technique where a pluralityof scanning lines are written with a plurality of laser beams in onescanning sequence was developed. This exposing technique is referred toas a multi-beam exposing technique.

To obtain a plurality of laser beams, a plurality of gas laser (forexample, He—Ne) oscillators are used as light sources. Alternatively, atechnique where a laser beam generated by one oscillator is time-dividedinto a plurality of portions by an acousto-optical modulator (AOM) orthe like was developed. However, as a technique for simplifying theapparatus and for reducing the size thereof, as disclosed in JapanesePatent Laid-Open Publication Serial No. SHO 54-7328, a technique where asemiconductor laser array integrally having a plurality of lightemitting portions on one device is used as a light source has beenproposed. This technique is referred to as a multi-beam laser scanningtechnique.

However, when a plurality of laser beams with parallel optical axes areentered into a collimator lens, the optical axes tend to spread out withlarge angles. Thus, the size of the reflecting surface of the deflectingunit and the size of the lens constructing the optical system becomevery large in comparison with the technique where only one laser beam isused.

FIG. 34 is a sectional view showing an optical path from a semiconductorlaser array to a image bearing member according to the multi-beam laserscanning technique. Now, for simplicity, consider a multi-beam laserscanning optical system using two laser beams, a convex collimator lens,and a convex image forming lens. The two laser beams spaced apart by δand emitted from a semiconductor laser array 321 are collimated by acollimator lens 302 of a focal length fc. Since the semiconductor laserarray 321 is disposed at an object side focal point of the collimatorlens 302, the two laser beams are intersected at an image side focalpoint F thereof. When a spot 306 of d₀=100 μm is formed on an imageplane 311, if fi is 200 mm, the diameter Wc of the beam entered into thescanning lens, that is, the collimate diameter, is given by the formula(8). The spot diameter or the beam diameter is a diameter where theintensity of the cross section of a beam is the power of the peakintensity times (1/e²). The distribution of this intensity accords withthe Gaussian distribution. $\begin{matrix}{{WC} = {d_{0}\sqrt{1 + ( \frac{4\quad \lambda \quad {fi}}{\pi \quad d_{0}^{2}} )^{2}}}} & (8)\end{matrix}$

However, λ is the wavelength of the laser, which is 780 nm. Thus, inthis example, the collimate diameter Wc is approximately 2 mm.

In the so-called edge emitting type laser diode which has beenconventionally used, as shown in a conceptual schematic diagram of FIG.35, the beam spread angle of a plane which includes the optical axis andwhich is in parallel with the contact surface largely differs from thatof a plane which includes the optical axis and which is perpendicular tothe contact surface. In the case of the conventional semiconductorlaser, the spread angle θp on a plane in parallel with the contactsurface is approximately 10 degrees in full width at half maximum.However, on a plane perpendicular to the contact surface, due to aninfluence of diffraction, the spread angle θt becomes approximately 30degrees in full width at half maximum. In addition, it is difficult tofreely set the spread angles θt and θp and the ratio thereof (namely,the ratio of longer diameter and shorter diameter of the ellipse of thebeam). To effectively use most of the radiated beam, it is necessary toraise the coupling efficiency of the semiconductor laser array and thecollimator lens. To obtain the collimate diameter of 2 mm, the focallength fc of the collimator lens 2 should be approximately 3 mm. On theother hand, in the conventional semiconductor laser array, the distanceδ of each adjacent light emitting portion cannot be set to 100 μm orless due to the mutual interference thereof.

The distance from the image side focal point F of the collimator lens302 to the reflecting surface 308 of the deflecting unit should bespaced apart by h due to the presence of each element of the scanningunit. On the other hand, when the distance between most spaced two of aplurality of light emitting portions on the semiconductor laser array321 is δmax, the distance q of these two beams on the reflecting surface308 of the deflecting unit can be expressed by the following formula.$\begin{matrix}{q = {\delta \quad {\max \cdot \frac{h}{fc}}}} & (9)\end{matrix}$

For example, when the number of beams is four and the distance of eachadjacent light emitting portion disposed in line on the semiconductorlaser array is 0.1 mm, δmax becomes 3×δ=0.3 mm. When the distance h fromthe image side focal point F of the collimator lens 302 to thereflecting surface 308 is 50 mm, q becomes 5 mm. Thus, the size of thereflecting surface should be at least the value where the distance q andthe laser beam collimator diameter are added. In other words, the sizeof the rotating portion of the deflecting unit becomes large and therebythe bearing is exposed to a large load. In addition, the deflecting unitis unbalancedly rotated. According to the formula (9), as the value offc/δmax decreases, the value of q increases.

Next, the case where the above-mentioned tilt angle compensation lenses307 and 307′ are added in the scanning optical system will beconsidered. Since the tilt angle compensation lenses are anamorphicoptical elements, the optical characteristics on the scanning planediffer from those on the sub scanning plane. As described above, sincethe tilt angle compensation lenses do not have optical powers on thescanning plane, by applying the above-mentioned formula (9), thedistance q of most spaced two beams on the reflecting surface can beobtained on the scanning plane. Thus, it is enough to consider only thesub scanning plane. FIG. 36 is a sectional view showing an optical pathon the sub scanning plane including the tilt angle compensation lens307. The figure shows elements disposed from the semiconductor laserarray 321 to the reflecting surface 308 of the deflecting unit.

As described above, on the sub scanning plane, each beam is imaged as aline image on the reflecting surface 308 of the deflecting unit. Eachline image is formed with a distance q on the right of the reflectingsurface in the sub scanning direction. When the focal length of the tiltangle compensation lens 307 on the left of the beam deflecting unit isft, the distance from the image side focal point F of the collimatorlens 302 to the tilt angle compensation lens 307 is t1, and the distancefrom the tilt angle compensation lens 307 to the reflecting surface 308of the deflecting unit is t2, provided that symbols in FIG. 34 are used,the mutual distance q′ of most spaced two of laser beams entered intothe tilt angle compensation lens 307 and the mutual distance q of thesebeams which are entered into the reflecting surface 308 are given by thefollowing formulas (10) and (11), respectively. $\begin{matrix}{q^{\prime} = {\delta \quad {\max \cdot \frac{t1}{fc}}}} & (10) \\{q = {{- q^{\prime}} \cdot \frac{{{{- {ft}} \cdot {t1}} + {{t2} \cdot {t1}} - {fc}}{\cdot {t2}}}{{ft} \cdot {t2}}}} & (11)\end{matrix}$

Generally, to allow a collimated laser beam to have a beam waist on thescanning plane, the relation of ft=t2 should be satisfied.

When t1<ft, two beams are not intersected on the imageside. Thus, therelation of q>q′ takes place. As the value of t1 decreases, the value ofq increases. For example, when t1 is 20 mm, ft is 30 mm, and t2 is 30mm, then q′ becomes 2 mm and q becomes 3 mm. The formula (11) containsq′. By applying the formula (10), as the value of fc/δmax becomes large,the value of q′ becomes small. At this point, since the relation oft1+t2=h is satisfied, the above-mentioned calculations can be applied onthe scanning plane. In other words, in this example, the distance ofmost spaced two of beams in the sub scanning direction is small.

This situation can also apply to the mutual distance of each ofplurality of laser beams which are entered into the scanning lens 4. Inother words, when the incident position of the above-mentioned tiltangle compensation lens is the incident position of the scanning lens,the formula (10) can be applied. The distance from the collimator lensto the scanning lens is larger than that of the above-mentioned example.Thus, the size of the scanning lens should be further increased.

Generally, the collimator lens has the highest optical power in theoptical elements which constructs the laser beam scanning opticalsystem. In other words, the focal length of the collimator lens is theshortest in the other constructional elements of the laser beam scanningoptical system. Thus, on the path where a plurality of laser beamsradiated from a semiconductor laser array reach a image bearing memberthrough an optical system, when the laser beams pass through thecollimator lens, the mutual angle of each laser beam is most largelychanged.

To prevent this problem, a technique for adding various optical elementswhich cause the positions of a plurality of laser beams reflected to benarrowed has been proposed. For example, in Japanese Patent Laid-OpenPublication Serial No. SHO 56-69611, an afocal optical system isdisposed behind a collimator lens so as to collect each beam on areflecting surface. However, the addition of such an optical systemresults in complicating the construction of the scanning optical system.Thus, it is not suitable from standpoints of cost, adjustment, andreliability.

In addition, as described above, when a plurality of laser beams travelon different optical paths, the optical system should be designed sothat the aberration and size of each image spot meet predeterminedvalues for each laser beam. Thus, the number of design steps increasesand the development period of the image forming apparatus becomes long.In addition, it is difficult to obtain designed solutions where all thelaser beams in any positions of the scanning region satisfy the designedspecifications. In other words, such requirements lead to a criticalproblem of an image forming apparatus with a high resolution or smalldiameter of image spots which requires advanced designing techniques.

Moreover, the laser scanning optical system which satisfies suchdifficult requirements should have larger reflecting surfaces of thescanning unit and larger effective diameters of the constructionallenses than those of the conventional laser beam scanning optical systemusing one laser beam. In addition, the construction of the former ismore complicated than that of the latter. In other words, the formerapparatus requires a large number of lenses and accurate adjustments oflens positions. Thus, the conventional production facilities cannot becommonly used.

4-2 Construction of Present Invention

FIG. 30 is a schematic diagram showing the overall construction of animage forming apparatus in accordance with the present invention. Theprocess for obtaining a print result on an image transfer material 351accords with a so-called electrophotographic process. As a image bearingmember 305 of an electrophotographic printer using a semiconductor laseras a light source, an organic photoconductor (OPC) with an increasedsensitivity in a longer wavelength region has been widely used. Thisimage bearing member 305 is charged to a predetermined surface potentialby a charger 352. Thereafter, a laser beam scanning unit 353 performs alight writing process, that is, a light exposing process. In accordancewith image information from the laser beam scanning unit 353, aplurality of laser beams 354 whose light intensities are individuallymodulated are scanned on image bearing member 305 in the axial directionthereof, thereby generating electric charges which neutralize thesurface potential only for the exposed portion. Thus, the absolute valueof the surface potential of this portion becomes low. As a result, onthe image bearing member 305, a distribution of surface potentials inaccordance with the image, that is, a static latent image is formed. Adeveloping unit 355 selectively adheres a developing agent in accordancewith the surface potentials to the image bearing member 305. Thus, thestatic latent image is developed. This developing agent is transferredto a transfer material 351 (normally, a paper) by a transferring unit356. The developing agent on the transfer material 351 is fixed with athermal pressure by a fixing unit 357. Thereafter, the transfer material351 is unloaded from the apparatus.

FIG. 29 is a schematic diagram showing the construction of a laser beamscanning optical system in accordance with the present invention. In thelaser beam scanning unit 353 shown in FIG. 30, the laser beams 354 arefolded back and downwardly emitted. However, in this example, theillustration of the laser beams is simplified. In the figure, laserbeams emitted from a plurality of light emitting portions 341 a of amonolithic semiconductor laser array 341 are collimated to laser beamswith a predetermined beam diameter by a collimator lens 302. The laserbeams are entered into a rotating polygon mirror 303. As the polygonmirror 303 rotates, these laser beams are deflected. The laser beamswhich pass through an image forming lens 304 are imaged at spots 306 onthe image bearing member 305. The lighting and the amount of light ofeach of the light emitting portions 341 a is discretely controlled by acontrol unit 360.

As a semiconductor laser array having such characteristics, it ispreferable to use a so-called surface light emitting type semiconductorlaser array. It is more preferable to use a surface light emitting typesemiconductor laser array with light emitting portions in which a groupII-VI compound semiconductor is embedded.

FIG. 31 is a sectional view showing one of the light emitting portionstwo-dimensionally disposed on a device substrate of the surface lightemitting type semiconductor laser array. As shown in the figure, on theGa—As device substrate 322, a semiconductor laminate reflecting layer322 is formed. The reflecting layer 322 is composed of several tenlayers of two types of Al—Ga—As compounds. On the reflecting layer 322,a clad layer 324, an active layer 325, a clad layer 326, and a contactlayer 327 are disposed, each of which is composed of Al—Ga—As compounds.On the contact layer 327, a SiO2 dielectric laminate reflecting layer328 is formed. On the entire rear surface of the Ga—As substrate, awindow-shaped electrode 329 is formed. In addition, in the periphery ofthe dielectric laminate reflecting layer 328, a window-shaped electrode330 is formed. Thus, all the parts formed on the Ga—As substrate composean optical resonator. A light beam which is generated on the activelayer 325 reciprocatively travels between the upper reflecting layer 327and the lower reflecting layer 323 in the direction perpendicular to thesurface of the device substrate. Thus, the light beam is oscillated. Asa result, the optical axis of the laser beam 331 is nearly perpendicularto the surface of the device substrate. In the periphery of the opticalresonator, a group II-VI compound semiconductor is embedded as anembedded layer 332. As a group II-VI compound semiconductor, it ispreferable to use a group II-VI compound which contains two, three, orfour elements selected from both group II elements Zn, Cd, and Hg andgroup VI elements O, S, Se, and Te. In addition, it is preferable tomatch the lattice constant of the compound with that of thesemiconductor layers composed of the clad layer 324, the active layer325, and the clad layer 326. Since the electric resistance of the groupII-VI compound semiconductor is very large, the current is effectivelyclosed in the optical resonator. In addition, since the refractive indexof the embedded layer 332 differs from that of the Al—Ga—Assemiconductor layer, the beam which travels in the optical resonator inthe direction just or nearly perpendicular to the surface of the devicesubstrate is totally reflected at the interface with the embedded layer332. Thus, the beam is effectively closed in the optical resonator.

Therefore, when such a semiconductor laser array is used, the laseroscillation starts with a very small amount of current in comparisonwith the conventional laser. In other words, the threshold value of thesemiconductor laser array is lower than that of the conventionalsemiconductor laser. Thus, the amount of lost heat is small. In FIG. 31,a diode is formed on the Ga—As device substrate 322. The light generatedin the active layer 325 reciprocally travels between the reflectionlayers 323 and 328. Thus, a laser oscillation takes place. A laser beam331 is emitted from the reflection layer 328 whose reflectance issmaller than that of the reflection layer 328 in the directionperpendicular to the surface of the device substrate.

In addition, since the sectional area (near field pattern) of the laserbeam emitting portions of the surface light emitting type semiconductorlaser array is larger than that of the conventional edge emitting typesemiconductor laser, the spread angles of the laser beams are small.Although the value of the spread angle just depends on the area of thelight emitting window, the area can be precisely controlled by anetching process or the like. Thus, the spread angle can be keptconstant. For example, a laser beam with a spread angle of approximately8 degrees in full width at half maximum can be satisfactorily obtained.Moreover, in the surface light emitting type semiconductor laser, sincecurrent and light can be effectively closed in an optical resonator ofthe laser, the amount of heat generated by each light emitting portioncan be decreased. Moreover, when a plurality of light emitting portionsare adjacently disposed, mutual optical, electrical, and thermalinterferences can be remarkably decreased in comparison with theconventional edge emitting type semiconductor laser array. Thus, thedistance of each adjacent light emitting portion can be decreased incomparison with the conventional semiconductor laser. In other words,the order of 50 μm can be accomplished.

As described in the paragraph of the related art, to obtain collimatedbeams with a diameter of 2 mm by using the above-mentioned surface lightemitting type semiconductor laser, the focal length fc of the collimatorlens should be approximately 8 mm. In addition, since the distance δ ofeach adjacent light emitting portion on the semiconductor laser array341 can be set to as narrow as 50 μm, in the case where four beams aredisposed in line, δmax becomes 150 μm. When the reflecting surface ofthe deflecting unit is disposed in the same position (against thecollimator lens) as the embodiment of the related art, the distance q ofeach adjacent beam reflection position on the reflecting surface becomes⅕ times the value of the embodiment of the related art. The value offc/δmax of the embodiment of the related art is approximately 10. On theother hand, according to the present invention, the value of fc/δmaxbecomes approximately 53. Like the embodiment of the related art, whenthe distance h from the collimator lens to the reflecting surface is 50mm, the distance q of each adjacent beam reflection position becomesapproximately 0.94 mm. This value is not large in comparison with thecollimate diameter Wc of the beams.

In particular, consider the case where the spot diameter on the imagebearing member is set to 50 μm so as to form an image with a much higherresolution. By applying the above-mentioned formula, the collimatediameter Wc is doubled (namely, approximately 4 mm). Thus, the focallength fc of the collimator lens is also doubled. The distance q of eachadjacent beam reflection position on the reflecting surface is halved.

When each beam is tracked, the distance of each adjacent beam in anyposition on the optical axis is much smaller than the collimatediameter. Thus, even if an optical system deals with a plurality oflaser beams, it is possible to design the optical system by consideringonly one typical beam. Thus, the design work of the laser scanningoptical system is very simplified. When the accuracy of the image spotsis not important, the conventional laser beam scanning optical systemwith one laser beam can be used as it is.

In addition, in the surface light emitting type semiconductor laser,since the light emitting portions can be disposed at any positions wherethey do not interfere each other, they can be two-dimensionally disposedon the device. The laser beams radiated from the light emitting portionsdisposed on the device substrate are magnified by the opticalmagnification M of the scanning optical system, the distance of eachadjacent light emitting portion being δ. Thereafter, the resultant beamsare imaged at spots on the image bearing member, the distance of eachadjacent spot being δ′. The value of M is nearly equal to the ratio ofthe focal length of the collimator lens and that of the scanning lens.

Now, in an exposing system which scans the image bearing member withfour laser beams, consider the relation between scanning lines and imagespots. In addition, assume the case where four adjacent scanning linesare drawn with one scanning sequence. Now, the distance of most spacedtwo of image spots is represented with δ′max. When the image spots aredisposed as shown in FIG. 32(a), the value of δ′max can be decreased incomparison with the case where they are disposed in line as shown inFIG. 32(b). The positions of the image spots on the image bearing memberare similar to those of the light emitting portions on the semiconductorarray. Alternatively, when the tilt angle compensation optical system isused, a mapping relation where the sub scanning direction is multipliedby a particular magnification takes place. Thus, in the same opticalsystem, when the value of δ′max is small, the value of δmax is alsosmall. Thus, in the arrangement shown in FIG. 32(b), the value of q inthe formula (9), (10), or (11) becomes small. Thus, the size of thereflecting surface of the deflecting unit can be accordingly decreased.As a result, the effect of the present invention can be furtherimproved.

In the above example, the case where four laser beams are used wasconsidered. When the number of laser beams is further increased, thelight emitting portions can be freely disposed on the semiconductorlaser array so that the spots are disposed at the closest positions onthe image bearing member. As a result, more significant effects can beobtained than the above-mentioned construction. In FIG. 32(c), anexample of the arrangement of image spots according to scanning line inthe case where eight laser beams are used is shown. In other words, byapplying the formula (9), when the light emitting portions are disposedin line, δmax=7×δ. However, in the arrangement shown in FIG. 32(c), thevalue of q can be calculated substantially as δmax=3×δ in designing theoptical system. Thus, the effects of the present invention can befurther improved. In addition, since the scanning direction of the imagespot 306 a is the same as that of the image spot 306 e, thecorresponding light emitting portions can be driven in the same timing.

The focal length of a collimator lens in the case where a surface lightemitting type semiconductor laser array is used is larger than that inthe case where the conventional edge emitting type semiconductor laseris used. Thus, the allowance of the distance between the semiconductorlaser array and the collimator lens in the direction of the optical axiscan becomes large. As a result, the adjustment work in the productionstage can be simplified. In addition, the collimator lens can have aresistance to the deviation of its position due to temperature variationand aged tolerance.

As described above, according to the image forming apparatus of thepresent invention, a plurality of laser beams radiated from asemiconductor laser array are collimated by a collimator lens. Theresultant laser beams are deflected by a beam deflecting unit. Theresultant laser beams are imaged as spots on a image bearing memberthrough a scanning lens. Thus, an optical writing operation isperformed. The focal length fc of the collimator lens of the presentinvention is larger than that of the embodiment of the related art. Inaddition, the distance δ of each adjacent light emitting portion on thesemiconductor laser array is small. In particular, when the surfacelight emitting type semiconductor laser array is used, since the spreadangles of radiated laser beams are small, the focal length fc of thecollimator lens becomes long. Moreover, since the amount of heatgenerated in each light emitting portion is small and the electric andoptical interferences of thereof are small, the distance thereof can befurther decreased.

In the case where a tilt angle compensating lens is not used, thedistance q of most spaced laser beams disposed in line on the reflectingsurface of the deflecting unit is given by the above-mentioned formula(9).

In the case where the tilt angle compensating lens is used, the distanceq′ of most spaced beams on the tilt angle compensating lens and thedistance q of that on the reflecting surface of the deflecting unit aregiven by the formulas (10) and (11), respectively. By applying theformulas (9) and (10), it is found that q and q′ are reverselyproportional to fc/δmax. Moreover, by applying the formula (11), since qis proportional to q′, they are also reversely proportional to fc/δmax.In this case, on the scanning plane, the formula (9) can be applied.

In other words, when the inverse number of fc/δmax is multiplied by thevalue equivalent to the size in the direction of the optical axis, thedistance of each beam in the direction perpendicular to the optical axiscan be obtained. Generally, in a small image forming apparatus which canprint data on papers in A4 or similar size, the distance of eachadjacent optical element in the direction of the optical axis and thefocal length thereof are approximately 50 mm. This value is representedwith Z. On the other hand, when converted from the resolution, thecollimate diameter of the laser beams is approximately 2 mm. When themaximum distance of most spaced two of beams on each lens surface andthe reflecting surface is limited to the collimate diameter, the valueof δmax/fc×Z is preferably 2 mm or less. Thus, the value of fc/δmax ispreferably 25 or more.

In addition, when a plurality of laser beams are entered into thescanning lens and the distance of most spaced two of beams is 2 mm, thevalue of Z should be approximately 100 mm. Thus, according to theabove-mentioned calculation, the value of fc/δmax is preferably 50 ormore.

As described above, when the maximum value of the distance of eachadjacent beam on each lens and the reflecting surface is nearly the sameas the value of the collimate diameter of the laser beams, the sizes ofeach lens and the reflecting surface are not remarkably larger than thesize of an optical system which scans one laser beam. In addition, whenthe distance of each adjacent laser beam is smaller than the collimatediameter, a plurality of laser beams can be substantially treated as onelaser beam in designing the optical system.

The above-mentioned embodiment is only one embodiment of the presentinvention. When the spread angle of each beam is small and the distanceof each adjacent light emitting portion is small, the same effects canbe obtained. In addition, it should be noted that even if the rotatingmirror is constructed and/or produced by using a different method suchas a plastic injection molding method, the similar effects of thepresent invention can be attained. Moreover, besides the rotatingpolygon mirror as the deflecting unit, with a galvano mirror or thelike, the same effects can be similarly acquired.

Further, the construction of the surface light emitting type laserdescribed in the above-mentioned embodiment is only an example which canbe accomplished. When the above-mentioned relation of the focal distanceof the collimate lens and the distance of each adjacent light emittingportion is satisfied, the same effects of the present invention can beobtained regardless of the construction thereof.

It is obvious that the scope of the applications of the image formingapparatus of the present invention includes facsimile machines anddisplay units as well as printing apparatus such as the printers andcopy machines. In these apparatuses and units, the same effects of thepresent invention can be attained.

4-3 Effects

As described above, according to the image forming apparatus of thepresent invention, in an exposing technique using a plurality of laserbeams, with a semiconductor laser array which satisfies predeterminedconditions with respect to the focal length of a collimator lens and thedistance of each adjacent light emitting portion, the size of thereflecting surface of a scanning unit and the effective diameter of eachlens can be decreased without necessity of adding auxiliary opticalelements. Thus, the size of the scanning optical system or the imageforming apparatus can be reduced and thereby the cost thereof can bedecreased.

In addition, since a plurality of laser beams travel on almost the sameoptical path, a scanning optical system can be designed in the samemanner as that with one laser beam. Thus, the number of designing stepsof the system can be remarkably reduced and the developing periodthereof can be shortened. Moreover, the scanning optical system with onelaser beam can be used. Thus, the producibility is remarkably improved.

Further, when a plurality of laser beams enter each optical system whichconstructs the scanning optical system, provided that the distance ofmost spaced two of beams is shorter than the collimate diameter of thelaser beams, the conventional scanning optical system with one laserbeam can be used as it is. In other words, without any modification ofthe scanning optical system of the image forming apparatus using onelaser beam, by increasing the number of laser beams, an image formingapparatus with a high speed can be produced. Thus, unexpected benefitscan be obtained in producing the products.

Moreover, since the spread angles of laser beams are small, the distancebetween the collimator lens and the semiconductor laser array can beincreased. The adjustment allowance in the direction of the optical axisof the collimator lens is increased. Thus, as well as the increase ofproducibility, without influences of aged deterioration and temperaturechange, images with a predetermined spot diameter can be exposed. As aresult, the image quality is improved.

Section 5 Fifth Embodiment of Image Forming Apparatus

5-1 Comparison with Related Art

Before describing a fifth embodiment of the present invention, so as toeasily understand the conception thereof, the related art thereof willbe described.

A scanning optical system of a conventional image forming apparatus isdisclosed in for example Japanese Patent Laid-Open Publication HEI3-248114 (see FIG. 41). In the figure, as a light source, an edgeemitting type semiconductor laser array 420 where the center axis ofemitted beams is nearly in parallel with the surface of a devicesubstrate is used. The edge emitting type semiconductor laser array 420is disposed on the object side focal plane of a collimator lens 402. Atthe image side focal position of the collimator lens 402, an aperturestop 403 is disposed.

However, in the above-mentioned related art, since the beam spreadangles of beams of the edge emitting type semiconductor laser and thedistance of each adjacent light emitting portion thereof are large, theposition at which the cross sections of a plurality of beams are matchedis limited to a very near position of the image side focal position ofthe collimator lens. In particular, a large number of beams are disposedin line, the position at which the cross sections of the most spacedbeams are matched is further limited to a narrow region. Thus, theposition of the aperture stop is limited to a much narrow region. Inother words, the degree of freedom of designing a scanning opticalsystem becomes low.

In addition, since the optical system generally uses the holding frameof a lens as an aperture stop. In this case, it is not necessary toprovide an independent aperture stop. However, in the above-mentionedrelated art, since the cross sections of the plurality of beams are notmatched at the position of the collimator lens, the holding frame of thecollimator lens cannot be used as the aperture stop.

5-2 Construction of the Present Invention

FIG. 38 is a schematic diagram showing the construction of an imageforming apparatus of the present invention. Next, an image formingprocess of an electrophotographic printer will be described. A charger451 applies an equal electric charge to a image bearing member 407. Ascanning optical system 452 exposes and scans the image bearing member407, thereby forming a latent image thereon. Thereafter, a developingunit 453 adheres a developing agent on the latent image so as to form avisible image. An image transferring unit 454 transfers the developingagent, which constructs the visible image, on a transfer material 455such as a paper. Thereafter, a fixing unit 456 heats and melts thedeveloping agent and fixes it on the transfer material 455.

FIG. 37 is a schematic diagram showing the construction of a scanningoptical system of the embodiment of the present invention. The lightsource of the scanning optical system is a surface light emitting typesemiconductor laser array 401 where the center axis of emitted beams isnearly perpendicular to the surface of a device substrate 422. Aplurality of beams emitted from light emitting portions 401 a on thesurface light emitting type semiconductor laser array 401 are collimatedby a collimator lens 402. The resultant beams enter a deflecting surface405 of a rotating polygon mirror 404 which is a deflecting unit throughan aperture stop 403. As the polygon mirror 404 rotates, reflected beamsare deflected and scanned. The resultant beams are imaged on the imagebearing member 407 through an image forming lens 406.

The lighting and the amount of light of each light emitting portion 401a are individually controlled by a control unit 460.

Regardless of the edge emitting type and the surface light emittingtype, the spread angles of output beams of a semiconductor laser deviateto some extent depending on a light emitting portion for use. Inaddition, the diameters of beams collimated by the collimator lens 402also deviate. However, when the aperture stop 403 is disposed at theposition where the cross sections of the beams are nearly matched andthe diameter of the aperture stop 403 is almost the same as or slightlysmaller than the diameter of each collimated beam, the diameter of eachcollimated beam which passed through the aperture stop 403 becomesequal. As a result, the diameter of each spot imaged on the imagebearing member 407 becomes equal. Thus, a stable and high printingquality can be obtained. The printing quality does not deviate fromproduct to product. The beam spread angles and the diameters of beamsare represented as an intensity distribution of cross sections of beams.The intensity distribution accords with a Gaussian distribution. Thebeam spread angle represents a full width of the angle at which thecenter intensity is halved. On the other hand, the spot diameterrepresents a position where the intensity becomes 1/e² times the centerintensity.

Next, an effect where the beam diameters are equalized by an aperturestop will be described. When laser beams are limited by an aperturestop, they are diffracted as a characteristic of wave optics. When thecenter of the aperture stop accords with the center of an incident beamand the effect of diffraction is considered, the diameter d0 of an imagespot on the image bearing member can be expressed by the followingformula. $\begin{matrix}{d_{0} = \frac{k\quad \lambda \quad f}{D}} & (12)\end{matrix}$

where k is a constant; λ is the wavelength of a laser beam; f is thefocal length of the image forming lens; and D is the diameter of theaperture stop. When the ratio of the diameter d of a beam entered intothe aperture stop and the diameter D of the aperture stop is representedas a cutting ratio T=d/D, the constant k can be calculated by thefollowing formula (refer to “LASER & OPTICS GUIDE II”, JAPAN MellesGriot K.K). $\begin{matrix}{k = {1.6449 + \frac{0.6460}{( {T - 0.2816} )^{1.821}} - \frac{0.5320}{( {T - 0.2816} )^{1.891}}}} & (13)\end{matrix}$

As an example, when the diameter D of the aperture stop is 1 and thediameter d of each incident beam deviates in the range of 1±20%, thedeviation of the diameter of each image spot is limited in the rangefrom −3.1% to +5.9%. Thus, the aperture stop has an effect fordecreasing the deviation of the diameter of each image spot.

When the aperture stop is disposed at a position where cross sections ofa plurality of beams on the optical path are matched at least partiallyand where powers of all beams except for a beam with the highest powerwhich passed through the aperture stop become 90% or more of the beamwith the highest power, the deviation of powers of the beams can belimited to 10% or lower. With this deviation, a high printing qualityfree of uneven density can be obtained.

Next, a position of the aperture stop 403 which satisfies theabove-mentioned conditions will be specifically described with a systemshown in FIG. 39. FIG. 39(a) is a side view showing a scanning opticalsystem. FIG. 39(b) is a sectional view at the position of the aperturestop 403. In the surface light emitting type semiconductor laser array401, the spread angles of laser beams emitted therefrom can be set to 10degrees of less and the distance of each adjacent light emitting portioncan be set to 0.05 mm or less. In FIG. 39, the distance Δ between lightemitting portions A410 and B411 is set to 0.05 mm and the light emittingportion A410 is disposed on an optical axis 412 of the optical system.Beams emitted from the light emitting portions A410 and A411 arereferred to as beams A413 and B414, respectively. When the spread angleθ of each radiated beam is 10 degrees; the focal length fc of thecollimator lens 402 is 10 mm; and the distance h from the collimatorlens 402 to the deflecting surface 405 of the rotating polygon mirror is100 mm, then the diameter d of the beams becomes 3.0 mm. Now, assumethat the center of the aperture stop 403 is present on the optical axis412 and that the diameter D of the aperture stop 403 is equal to thebeam diameter d.

When the intensity distribution of the cross section of a laser beamaccords with a Gaussian distribution, the intensity I of the laser beamcan be expressed by the following formula. $\begin{matrix}{I = {\frac{2P}{\pi \quad w^{2}}\exp \{ {{- 2}( \frac{x}{w} )^{2}} \}}} & (14)\end{matrix}$

where P is the entire power of a beam; w is the radius of the beam; andx is the distance f rom the center of the beam. The power of the beamA413 which passed through the aperture stop 403 can be obtained byintegrating the above formula as the formula (15). $\begin{matrix}{\int_{0}^{W}{2\quad \pi \quad x\quad I{x}}} & (15)\end{matrix}$

The resultant value is 86.5% of the power P of which the beam does notpass through the aperture stop 403. Thus, the power of the beam B414should be 77.9% or more of the power where the beam passed through theaperture stop 403 because of 86.5×0.9=77.9(%). Next, the power of thebeam B414 which passed through the aperture stop 403 will be obtained.When the distance between the center axis of the beam B414 and theoptical axis 412 on the plane of the aperture stop 403 is t, the crosssectional intensity I of the beam B414 which has not yet passed throughthe aperture stop 403 can be expressed with a cylindrical coordinatesystem by the following formula. $\begin{matrix}{I = {\frac{2P}{\pi \quad w^{2}}{\exp \lbrack {{- \frac{2}{w^{2}}}\{ {( {{r\quad \cos \quad \varphi} - t} )^{2} + ( {r\quad \sin \quad \varphi} )^{2}} \}} \rbrack}}} & (16)\end{matrix}$

where r is the distance from the optical axis 412 to any point A on theaperture stop 403; and φ is the angle made of a plane B, which isproduced by the optical axis 412 and the center axis 414 a of the beamB414, and the line where the optical axis 412 and the point A areconnected. The power of the beam B414 which passed through the aperturestop 403 can be obtained by integrating the above formula by thefollowing formula. $\begin{matrix}{\int_{0}^{2\quad \pi}{\int_{0}^{W}\quad {I{x}{\varphi}}}} & (17)\end{matrix}$

When the ratio of t and the beam diameter d is 0.200, the power of thebeam which passed through the aperture stop 403 becomes 77.9% of thepower P which has not yet passed through the aperture stop 403. Thus,the same result is obtained. In other words, when the aperture stop 403is disposed at a position where t/d becomes 0.200 or less, thedifference of the powers of the beams A413 and B414 which passed throughthe aperture stop 403 becomes 10% or less. When the aperture stop 403 isdisposed at the position of the collimator lens 402 or at the positionof the deflecting surface 405 of the rotating polygon mirror, therespective distances t are 0.05 mm and 0.45 mm and the respective valuesof t/d are 0.017 and 0.15. In each case, t/d is less than 0.200. Thus,the aperture stop 403 can be disposed at any position midway between thecollimator lens 402 and the deflecting surface 405 of the rotatingpolygon mirror. As a result, the degree of freedom of designing theoptical system can be increased. In addition, even if the number ofbeams which are disposed in line is increased, the aperture stop 403 canbe disposed at the position of the collimator lens 402. Thus, theholding frame of the collimator lens 402 can be used as the aperturestop. Since an independent aperture stop is not necessary, the number ofconstructional elements can be decreased.

Although in the embodiment the aperture stop was disposed midway betweenthe collimator lens and the rotating polygon mirror, provided that theabove-mentioned conditions are satisfied, the aperture stop can bedisposed midway between the surface light emitting type semiconductorlaser array and the collimator lens. In addition, the optical systemdisposed midway between the light source and the rotating polygon mirroris not limited to the collimator lens which collimates beams.

FIG. 40 is a schematic diagram showing the construction of elementsdisposed from a semiconductor laser to a rotating polygon mirror asanother embodiment. An aperture stop 403 is disposed at a position wherethe center axes of a plurality of beams are intersected with an opticalaxis 412.

In this construction, on the plane of the aperture stop 403, the centeraxes of the beams are completely matched each other. Thus, theregularity of the diameters of image spots and that of powers thereofare improved in comparison with the above-mentioned embodiment. Whenhigher characteristics are required for image spots, this constructioncan be also used. In this embodiment, the optical system disposed midwaybetween the light source and the rotating polygon mirror is not limitedto the collimator lens which collimates beams.

5-3 Effects

As described above, according to the present invention, since theaperture stop is disposed at the position where the cross sections of aplurality of beams are nearly matched on the optical path midway betweenthe surface light emitting type semiconductor laser array and thedeflecting unit, even if the spread angles of the beams deviate, thediameters of the image spots can be kept equal. Thus, a stable and highprinting quality can be obtained. In addition, since the aperture stopcan be disposed at any position in a wide range on the optical pathmidway between the semiconductor laser array and the deflecting unit orsince the holding frame of the collimator lens can be used as theaperture stop, the degree of freedom of designing the optical system canbe increased.

Section 6 Sixth Embodiment of Image Forming Apparatus

6-1 Comparison with Related Art

Before describing a sixth embodiment of the present invention, so as toeasily understand the conception thereof, the related art thereof willbe described.

A scanning optical system of an image forming apparatus using asemiconductor laser array which has been conventionally used isdisclosed in for example Japanese Patent Laid-Open Publication SerialNo. HEI 3-248114. semiconductor laser array 501 is disposed on an objectside focal position of a collimator lens 502. At the image side focalposition of the collimator lens 502, an aperture stop 503 is disposed.

However, in this related art, since the position of the aperture stop islimited to the image side focal position of the collimator lens, thedegree of freedom of designing an optical system is low. Moreover, in aconventional optical system, a holding frame of a lens is sometimes usedfor an aperture stop. However, according to the related art, such aconstruction cannot be accomplished.

6-2 Construction of the Present Invention

FIG. 48 is a schematic diagram showing the construction of an imageforming apparatus of the present invention. Next, this image formingprocess will be described. A charger 551 applies an equal electriccharge to a image bearing member 507. A scanning optical system 552exposes and scans the image bearing member 507, thereby forming a latentimage thereon. Thereafter, a developing unit 553 adheres a developingagent on the latent image so as to form a visible image. An imagetransferring unit 554 transfers the developing agent, which constructsthe visible image, on a transfer material 555 such as a paper.Thereafter, a fixing unit 556 heats and melts the developing agent andfixes it on the transfer material 555.

FIG. 42 is a schematic diagram showing the construction of a scanningoptical system of the embodiment of the present invention. A pluralityof beams emitted from a semiconductor laser array 501 are collimated bya collimator lens 502. The resultant beams enter one of deflectingsurfaces 505 of a rotating polygon mirror 504 which is a deflecting unitthrough an aperture stop 503. As the polygon mirror 504 rotates, emittedbeams are deflected and scanned. The resultant beams are imaged on theimage bearing member 507 through an image forming lens 506.

In FIGS. 42 and 49, the aperture stop 503 is disposed on the opticalpath midway between the semiconductor laser array 501 and the deflectingunit 504. When the focal length of the collimator lens 502 is f; thedistance from the deflecting unit 504 side focal point of the collimatorlens 502 to the aperture stop 503 is s; the distance from the opticalaxis of the collimator lens 502 to the optical axis of a light emittingportion which is most spaced from the optical axis of the collimatorlens 502 is t; the diameter of the aperture stop 503 is D; and thediameter of a collimated beam is d, then the relations given by thefollowing formulas can be obtained. $\begin{matrix}{\frac{st}{f} \leq {{0.12\quad ( \frac{D}{d} )^{2.3}} + 0.17}} & (18) \\{\frac{D}{d} \leq 2} & (19)\end{matrix}$

Alternatively, instead of the formulas (18) and (19), the relationsgiven by the following formulas can be obtained. $\begin{matrix}{\frac{st}{f} \leq {{0.06\quad ( \frac{D}{d} )^{2.9}} + 0.08}} & (20) \\{\frac{D}{d} \leq 2} & (21)\end{matrix}$

Generally, the spread angles of output beams of a semiconductor laser501 deviate to some extent depending on a light emitting portion foruse. In addition, the diameters of beams collimated by the collimatorlens 502 also deviate. However, when the aperture stop 503 is disposedat the position where the cross sections of the beams are nearly matchedand the diameter of the aperture stop 503 is almost the same as orslightly smaller than the diameter of each collimated beam, the diameterof each collimated beam which passed through the aperture stop 403becomes equal. As a result, the diameter of each spot imaged on theimage bearing member 505 becomes equal. Thus, a stable and high printingquality can be obtained. The printing quality does not deviate fromproduct to product. The beam spread angles and the diameters of beamsare represented as an intensity distribution of cross sections of beams.The intensity distribution accords with a Gaussian distribution. Thebeam spread angle represents a full width of the angle at which thecenter intensity is halved. On the other hand, the spot diameterrepresents a position where the intensity becomes 1/e² times the centerintensity.

Next, an effect where the beam diameters are equalized by an aperturestop 503 will be described. When laser beams are limited by the aperturestop 503, they are diffracted as a characteristic of wave optics. Whenthe center of the aperture stop accords with the center of an incidentbeam and the effect of diffraction is considered, the diameter d0 of animage spot on the image bearing member 505 can be expressed by thefollowing formula. $\begin{matrix}{d_{0} = \frac{k\quad \lambda \quad f}{D}} & (22)\end{matrix}$

where k is a constant; λ is the wavelength of a laser beam; f is thefocal length of the image forming lens; and D is the diameter of theaperture stop. When the ratio of the diameter d of a beam entered intothe aperture stop and the diameter D of the aperture stop is representedas a cutting ratio T=d/D, the constant k can be calculated by thefollowing formula (refer to “LASER & OPTICS GUIDE II”, JAPAN MellesGriot K.K). $\begin{matrix}{k = {1.6449 + \frac{0.6460}{( {T - 0.2816} )^{1.821}} - \frac{0.5320}{( {T - 0.2816} )^{1.891}}}} & (23)\end{matrix}$

As an example, when the diameter D of the aperture stop is 1 and thediameter d of each incident beam deviates in the range of 1±20%, thedeviation of the diameter of each image spot is limited in the rangefrom −3.1% to +5.9%. Thus, the aperture stop has an effect fordecreasing the deviation of the diameter of each image spot.

The distribution of intensity of the cross sections of beams collimatedby the collimator lens 502 nearly accords with a Gaussian distribution.As shown in FIG. 44, when a collimated beam passes through the aperturestop 503, the periphery of the beam is vignetted. Thus, the power of thebeam which passed through the aperture stop 503 decreases. As shown inFIG. 45, when the aperture stop 503 is disposed at the position of thedeflecting unit side focal point of the collimator lens 502, since allthe center axes of the beams which passed through the aperture stop 503are matched, the power decrease of each beam which passed through theaperture stop 503 is equal. However, when the position of the aperturestop 503 is limited to the position of the deflecting unit side focalpoint of the collimator lens 502, the degree of freedom of designing theoptical system decreases.

As shown in FIG. 46, when the aperture stop 503 is disposed at aposition apart from the deflecting unit side focal point of thecollimator lens 502, the center axes of a plurality of beams whichpassed through the aperture stop 503 are not matched each other. Thus,when a beam 511 a which travels along an optical axis 510 of thecollimator lens 502 and a beam 511 b which travels with an inclinationto the optical axis 510 pass through the aperture opening 503, they arevignetted in different manners. FIG. 47 shows the difference of thevignetting manners. FIG. 47(a) shows the vignetting manner of the beam511 a, while FIG. 47(b) shows the vignetting manner of the beam 511 b.In these figures, the portions of intensity distributions of crosssections of beams which are vignetted by the aperture stop are hatched.The powers of beams which passed through the aperture stop 503 differone by one. In this condition, the power of the beam 511 a is largerthan that of the beam 511 b. When the difference between these powersexceeds a predetermined level, uneven density takes place on a printedpaper. However, when this difference is in a particular range, asubstantially good printing quality can be obtained. Thus, the allowablerange where the aperture stop can be disposed is present.

When the condition represented by the formula (18) is satisfied, thefluctuation of powers of beams which passed through the aperture stop503 can be suppressed to 20% or less. However, since the formula (18) isan approximate formula, it is satisfied under the condition of theformula (19). On the other hand, when the condition represented by theformula (19) is satisfied, the fluctuation of powers of beams whichpassed through the aperture stop 503 can be suppressed to 5% or less.Likewise, the formula (20) is an approximate formula. Thus, this formulais satisfied under the condition of the formula (21).

According to an experiment conducted by the applicant of the presentinvention, when only letters and lines are printed with the imageforming apparatus, if the fluctuation of powers of image spots isapproximately 20% or less, a good printing quality can be obtained. Ifthe fluctuation is larger than 20%, the printing quality is degraded. Onthe other hand, when half tone patterns of graphic images or small dotsare printed, the fluctuation of powers of image spots results in unevendensity of printed images. To obtain a good printing quality, thefluctuation should be limited to approximately 5% or less. Thus, theconditions given by the formulas (18) and (19) are suitable for imageforming apparatuses which only print letters and lines. On the otherhand, the conditions given by the formulas (20) and (21) are suitablefor image forming apparatus which print not only letters, but half tonesand mesh dots.

According to the above-mentioned conditions, the aperture stop 503 canbe disposed at any position in a wide range on the optical path betweenthe semiconductor laser array 511 and the deflecting unit 503. Thus, thedegree of freedom of designing an optical system is increased.Alternatively, the holding frame of the collimator lens 502 can be usedas the aperture stop. In other words, since an independent aperture stopis not necessary, the number of constructional elements can bedecreased.

Next, a position of the aperture stop 503 which satisfies theabove-mentioned conditions will be specifically described with a systemshown in FIG. 49. Now, consider the case where a surface light emittingtype semiconductor laser array is used as the semiconductor laser array501. The surface light emitting type semiconductor laser is asemiconductor laser where the center axes of beams emitted are nearlyperpendicular to the surface of a device substrate. In the surface lightemitting type semiconductor laser array 501, the spread angles of laserbeams emitted therefrom can be set to 10 degrees or less and thedistance of each adjacent light emitting portion can be set to 0.05 mmor less. Now, consider the semiconductor laser array 501 having twolight emitting portions. The distance t between the light emittingportions 512 a and 512 b is set to 0.05 mm and the light emittingportion 512 a is disposed on an optical axis 510 of the collimator lens502. The lighting and the amount of light of the light emitting portions512 a and 512 b are controlled by a control unit 560 (see FIG. 42).Beams emitted from the light emitting portions 511 a and 511 b arereferred to as beams 511 a and 511 b, respectively. When the spreadangle θ of each radiated beam is 10 degrees; the focal length f of thecollimator lens 502 is 10 mm; and the distance h from the collimatorlens 502 to the deflecting surface 505 of the rotating polygon mirror is50 mm, then the diameter d of the beams becomes 3.0 mm. Now, assume thatthe center of the aperture stop 503 is present on the optical axis 510and that the diameter D of the aperture stop 503 is equal to the beamdiameter d.

Since D/d=1, both the conditions given by the formulas (19) and (21) aresatisfied. According to the condition given by the formula (18), sinces≦58 mm, the aperture stop 503 can be disposed at any position midwaybetween the collimator lens 502 and the deflecting surface 505 of therotating polygon mirror. On the other hand, according to the conditiongiven by the formula (20), since s≦28 mm, the aperture stop 503 can bedisposed at any position midway between the deflecting surface 505 sidefocal point of the collimator lens 502 and the position 28 mm right ofthe deflecting surface 505. In addition, even if the number of lightemitting portions which are disposed in line is increased and therebythe value of t becomes large, provided that the conditions given by theformulas (18) and (19) are satisfied, when the number of the lightemitting portions is 12 or less, the aperture stop 503 can be disposed.On the other hand, according to the conditions given by the formulas(20) and (21), when the number of light emitting portions is six orless, the aperture stop 503 can be disposed at the position of thecollimator lens 502. When the aperture stop 503 can be disposed at theposition of the collimator lens 502, since the holding frame of thecollimator lens 502 can be used as an aperture stop, an independentaperture stop is not necessary and thereby the number of constructionalelements of the scanning optical system can be decreased.

6-3 Effects

As described above, according to the present invention, an aperture stopis disposed on an optical path midway between a semiconductor laserarray and a deflecting unit in the conditions given by the formulas (18)and (19). Thus, even if the spread angles of the beams deviate, thediameters of the image spots can be kept equal. Thereby, a stable andhigh printing quality can be obtained. In addition to a high printingquality of letters and lines, an aperture stop can be disposed on anoptical path at any position in a wide range between a semiconductorlaser array and a deflecting unit. Alternatively, a holding frame of acollimator lens can be used as an aperture stop. As a result, the degreeof freedom of designing an optical system is improved.

Moreover, an aperture stop is disposed on an optical path midway betweena semiconductor laser array and a deflecting unit in the conditionsgiven by the formulas (20) and (21). Thus, even if the spread angles ofthe beams deviate, the diameters of the image spots can be kept equal.Thereby, a stable and high printing quality can be obtained. In additionto a high printing quality of not only letters and lines, but half toneimages and mesh points, an aperture stop can be disposed on an opticalpath at any position in a wide range between a semiconductor laser arrayand a deflecting unit. Alternatively, a holding frame of a collimatorlens can be used as an aperture stop. As a result, the degree of freedomof designing an optical system is improved.

INDUSTRIAL UTILIZATION

The image forming apparatus according to the present invention can printdata on papers at a high speed by using an electrophotographic process.The image forming apparatus can be used as an output unit of a computer,a facsimile machine, a multi-functional copy machine, and so forth.

What is claimed is:
 1. In an image forming apparatus, comprising: animage bearing member having a surface for forming a static latent imagethereon; and means comprising a laser beam scanning unit for scanningsaid surface of said image bearing member with a plurality of laserbeams to form said static latent image; the improvement wherein saidlaser beam scanning unit comprises: an array of light emitting portionsof one semiconductor substrate for emitting said laser beams from asurface of said semiconductor substrate and means for controllinglighting and amount of light emission of each of said light emittingportions individually, a collimator lens having a focal length fc forcollimating said laser beam; and means comprising a deflecting unit fordeflecting said collimated laser beams to said surface of said imagebearing member; wherein said light emitting portions are disposedtwo-dimensionally on said surface of said semiconductor substrate,whereby said laser beams provide respective spots that are disposedtwo-dimensionally on said surface of said image bearing member and runalong respective scanning lines; and wherein a distance between a mostspaced two of said plurality of light emitting portions is δ max, fc/δmax being 25 or more.